Two species of seaweed fly, Coelopa frigida (Fabricius) and Coelopa pilipes (Halliday) (both Diptera: Coelopidae: Coelopini), compete for resources within deposits of marine algae washed ashore on British beaches. Previous studies report that adult flies exhibit algal-specific behaviour that may influence interspecific interactions. It is predicted that coelopid larvae may also demonstrate algal-specific dietary preferences. Larval dietary preferences are investigated by comparing the ratios of 13C/12C and 15N/14N in both wild flies and macroalgae to those of laboratory-reared flies. Results showed only a small difference between the stable isotope ratios of the most abundant algae, Laminaria spp. (Laminariaceae) and Fucus spp. (Fucaceae), although there were significant differences between wild adult coelopids. This result illustrates different metabolic processes in two closely related species. The stable isotope ratios of wild-caught coelopids were found to differ significantly from laboratory-reared coelopids. This is either the result of red algae in the diet of natural populations or a difference in bacterial communities. We suggest that experiments with laboratory-reared flies/specimens can greatly increase the utility of stable isotope analysis in the investigation of animal food webs, even where potential diets are isotopically similar. However, this approach is dependent on re-creations that accurately mimic natural conditions.