This study analyses the institutional and organizational arrangements of irrigation waterrnmanagement and identifies the determinants of collective action and its effectiveness inrnmanaging communal irrigation schemes in the districts of Atsbi wemberta (Tigray region)rnand Ada'a (Oromiya region), Ethiopia. Results are based on data collected from a survey ofrn169 groups (communities) and 22 scheme level focus group interviews. All tabias which havernirrigation projects that operated, in year 2006107 are included. Analysis of descriptive andrneconometric methods are used. Analysis of qualitative information supplemented therneconometric results.rnOur result reveals that, in Atsbi, over 221.1 ha of land, there are 14 irrigation schemes whichrnare used by 1855 beneficiary households. On the other hand, in Ada'a there are a total of 2059rnirrigation water beneficiaries in 8 communal managed schemes, which irrigate 960.5 1ha ofrnland. Each irrigation scheme is a common property resource that is owned and managed byrnthe community. In both study areas, each scheme has its own water users association which isrnadministered by water users committee. The associations are local institutions which have arnbasic character of authority and by-laws. In addition, water users form groups at each outletrn(block) level for administrative purposes, which are managed by group (block) leaders. Theyrnare 94 ill Atsbi and 75 in Ada 'a. in each irrigation schemes, rotational irrigation is practicedrnbased on counting dates or complaints, but not based on the water need of plants.rnThe econometric results show that collective action is more effective in irrigation water usersrnof Atsbi than Ada'a. We also found evidence for all inverted V-shaped relationship betweenrnNumber of household beneficiaries and collective action. the findings also imply thatrncommunity irrigation water management can contribute to a more sustainable irrigation waterrnuse and as a re5ull ill increasing agricultural production and productivity. Collective actionrnfor irrigation water management may be more beneficial and more effective ill areas withrnintermediate number of beneficiaries, in areas that are close to market access, in groups thatrnhave longer years of experience in irrigation water use, groups with larger family sizes, inrncommunities with greater number of local organizations, and in schemes where there wasrnparticipation of beneficiaries during construction of the scheme.rnThough women are found to be significantly involved in irrigation agriculture in male headedrnhouseholds, the revenue generated frol11 agriculture is controlled by men. On the other handrnwe found that the participation of female headed households at forum and leadership is veryrnlow. However, the estimation result shows that less number of conflicts and violation of rulesrnare associated with high proportion of female household heads. This suggests policyrnintervention is needed to encourage the participation of women in farm, forum and atrnleadership level in ,water users association and in conflict resolution committee. In additionrneffort should be made to change the ,wrong perception of the society towards genderrninequality.rnOur evidence also shows that instead of higher level of education status, it is greater numberrnof provision of training which favors collective action. Thus, expansion of training forrnbeneficiary farmers by governmental and non-governmental organizations will have a positivernand a significant impact on increasing efficiency water use collectively. In communities thatrnare more remote from markets or have larger number of beneficiaries, private-orientedrnapproaches to resource management may be more effective. Access to formal credit andrnextension programs have a positive significant impact on collective action. therefore,rnemphasis should be given on availability of such institutional support services. the presencernof external organizations reduce local efforts to enforce the restricted rules, suggesting thatrnthe roles of external organizations needs to be demand driven and complementary to localrninputs .rnGenerally, collective action in managing irrigation water functions well in both study woredas.rnIt is found that farm households have started to grow crops which were 1101 previously grownrnin the areas. It was also found that it has a positive impact on their income as well as on thernliving standard of their families. In addition, through time beneficiary farm householdsrndepend more all the production from their irrigated fields, which enabled them to harvestrnmore than once a year. Therefore, as beneficiary farmers shift to high value but perishablerncommodities, emphasis should be given to marketing extension, especially in facilitatingrnmarkets and marker linkages to farmers. Furthermore, through time the demand forrnirrigation water increases among beneficiary farmers. Therefore, assigning of water rightsrnand strengthening organization and operation of WUAs will be very essential for furtherrnefficient use of the common pool resource.