A clonal culture of the ciliate Strombidinopsis cheshiri Snyder and Ohman was established from British Columbian coastal waters. Using the solitary 4 μm diatom Thalassiosim pseudonana (Hustedt) Hasle and Heimdal as food, a numerical response was obtained by keeping the ciliates in semi-continuous culture, measuring specific growth rates at different prey concentrations, and fitting the data to a rectangular hyperbolic function; this provided growth and mortality rates. From the experiments four pieces of information were obtained for S. cheshiri: 1) the threshold concentration (i.e., the prey concentration where ciliate growth = 0) was ~ 103· ml−1 (~6 ng C · ml−1); 2) the major increase in growth rate occurred between 103–104 prey · ml−1 (~ 6–60 ng C · ml−1); 3) the maximum growth rate was μ = 0.985 (~ 1.4 divisions · day −1); and 4) the ciliate was capable of selfing conjugation. The growth rate data were, however, variable due to sub-clonal differences (i.e., clones secondarily isolated from the original clone). Data obtained from examining cell isolates suggested that the sub-clonal differences were caused by clonal ageing and selfing conjugation. Consequently, abnormally low growth rates were assumed to be due to ciliates with a high mutational load; outliers were removed and not included in the final numerical response. This is the first observation of selfing conjugation in oligotrichs and one of only a few observations of clonal ageing for these ciliates. The results suggest that 1) this genus may be useful for future work on oligotrich mating and 2) older cultures of these ciliates should be used with caution when estimating ecological parameters.