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We examined differences in digestibility and viability following gut passage through water penny larvae (Psephenus herricki) of Synedra ulna and Achnanthidium lanceoloatum, two common diatom taxa that differ in growth habit and autecological characteristics. Prior to the experiment, diatoms were cultured in Chu-10 media in petri plates to establish a monospecific biofilm to offer grazers. After collection, insects were left to clear their guts over night, allowed to graze for 3 hours on diatom biofilms, and then placed in vials over 1-mm mesh to defecate. Samples from source material and from insect feces were mounted in syrup media and the ratio of chloroplast-containing to empty diatom frustules was microscopically assessed. In addition, subsamples from source material and feces were sprayed onto agar plates prepared with Chu-10 and individual cells were mapped and tracked for 5 days to quantify reproduction. Cells of both S. ulna and A. lanceolatum taken from source material formed colonies on agar. Achnanthidium lanceolatum cells from insect feces also formed colonies, but with lower densities than those from source material. In contrast, none of the S. ulna cells tracked from fecal cultures formed colonies, and the percentage of S. ulna cells that were dead was significantly greater in feces relative to source material. Dead cell percentages of A. lanceolatum were also higher in feces relative to source material, but to a lesser degree than observed for S. ulna. These findings have potential implications for linking patterns of energy transfer in stream ecosystems and the structure and dynamics of benthic microalgal communities.