Throughout the year, algae were the most frequently encountered organisms in the gut of larvae of landlocked and anadromous sea lamprey (Petromyzon marinus) and American brook lamprey (Lampetra lamottei). Relative abundance of organisms in the gut of both species was similar and did not change with length of larvae. In general, relative abundance of organisms in the gut was similar to that in samples of sediment and water. Occasionally, filamentous and episammic forms were not ingested because of their larger size.The number of algal cells ingested per unit weight of sea lamprey decreased inversely with size of larvae. In the summer the number of algal cells in the gut was about thrice that found in the winter for larvae of comparable size. Survival among diatoms evacuated from the digestive tract varied from 45% in July (17.8 C) to 90% in January (0 C). Representatives of theAchnantheshad the lowest survival rate (10–20%), while those ofAmphoraandCyclotellahad the highest (65–100%). Complete evacuation required approximately 54 and 70 hr at 16.0 and 2.5 C, respectively. Evacuation rate did not differ among the organisms consumed. The numbers of algae consumed by larval sea lamprey in a stream relative to their estimated abundance was considered negligible.