Delta 13C values in particulate organic C were measured during batch growth of cultures of the phototroph Isochrysis galbana and the heterotroph Oxyrrhis marina. There was an increase in delta13C during growth of the phototroph which was reversed as the number of predators increased and/or the rate of C-fixation decreased. There was a diurnal change in lsochrysis delta13C with lower values at the end of the dark phase. Evidence of 12C concentration was apparent in mixed cultures, especially with stable predator and prey numbers where delta13C values decreased while particulate organic carbon remained constant. In cultures where the phototroph was effectively eliminated, delta13C values remained stable; changes in delta13C required the presence of the phototroph. Because delta13C values in mixed microbial populations can vary in response to various factors, the use of such values in trophic web analyses must be undertaken with caution. Also, care must be taken not to use high concentrations of N or P in batch cultures for either stock maintenance or experimental work because of the implications for C-stress with inadequate dissolved inorganic C (DIC).