Abstract: In a 12-week feeding trial, the effect of 2 algae meals (Ulva rigida or Cystoseira barbata) on feed intake, growth, and nutrient utilization of juvenile Nile tilapia, Oreochromis niloticus, was investigated. This was the first attempt to utilize Cystoseira barbata as a feed supplement for Nile tilapia. The fish were fed to apparent satiation with formulated diets supplemented with various levels of Ulva meal (5%, 10%, or 15%) or Cystoseira meal (5%, 10%, or 15%). A diet without algae meal served as a control diet. The highest values for weight gain were for fish fed the 5% Cystoseira diet, control diet, and 5% Ulva diet (156%, 151%, and 150%, respectively), but the values were not significantly different (P > 0.05) compared to other treatments, except for the fish fed on the 15% Ulva diet (P < 0.05), which exhibited the lowest weight gain. Fish fed the diet containing 15% Ulva meal showed the poorest feed conversion ratio (FCR). Protein and energy utilization tended to decrease in the groups fed the algae meals at the highest supplementation level of 15%. Carcass lipid levels decreased with increasing levels of Ulva meal, while an increase in carcass lipid level with increasing levels of Cystoseira meal was observed (P < 0.05). The results suggested that Ulva rigida or Cystoseira barbata meals could be used in small percentages in tilapia diets.