Fish and fish products are one of the food items that are of excellent nutritional source but it isrnhighly perishable unless well preserved. Oreochromis niloticus (Nile Tilapia) is the mostrnimportant fish species of Lake Tana fishery. Bacteriological load and physicochemical changesrnwere evaluated during the storage of Nile Tilapia fillets at -18ÂºC and 5ÂºC for 30 days. The fishrnsamples were collected from Lake Tana. Enumeration of Aerobic Colony Count,rnEnterobacteriaceae count and Psychrotrophic count; determination of protein, ash, and moisturerncontents, pH and TVB-N changes were undertaken. The analyses of all the parameters wererncarried out at regular 5 days interval on the fish fillets stored for up to 30 days. Data analysisrnwere performed using SPSS version16 and Microsoft excel 2010. There was significant increasernin Aerobic Colony Count, Enterobacteriaceae count and Psychrotrophic count on the filletsrnstored at 5ÂºC. The frozen condition showed significant effect on inhibiting all of the three groupsrnof microbial counts. The moisture, protein, fat, and ash contents of fresh Nile tilapia fish filletsrnwere 77.98%, 20.26%, 0.523% and 1.322%, respectively. The fillets stored at 5ÂºC showedrnsignificant decreases in protein and ash contents during 30 days of storage. On the other hand,rnthere were no significant changes in moisture and ash contents of fillets kept under frozenrncondition for about 30 days. However, there was a significant decrease in crude protein contentrnon the later days of storage under frozen condition. Moreover, TVB-N content of the fish filletsrnincreased from 3.92 to 45.04 mgN/ 100g and 5.37 mgN/100g on samples kept at 5ÂºC and -18ÂºC,rnrespectively. pH were also increased from 6.23 to 7.60 and 6.81 on the samples stored at 5ÂºCrnand -18ÂºC, respectively. The result shows that Nile Tilapia from Lake Tana had high proteinrncontent, low fat content, and high microbial load on fresh fillets, and freezing can keep thernqualities of the fish relatively for longer period within safe levels and refrigeration could be ablernto keep the qualities briefly.rnKey words: Bacteriological load, Fresh, Oreochromis niloticus, Refrigeration, and TVB-N.