To assess short-term variability in the community composition and community structure of tintinnid ciliates, herbivores of the microzooplankton. Location North-west Mediterranean Sea. We sampled on 18 dates over a 4-week period in 2004 at an openwater site. Species were classified as 'core species', found on every date, or 'occasional species', absent on one or more dates. Species abundance distributions of the entire community, and separately the core and occasional species, were compared with geometric, log-series and log-normal distributions. Core and occasional species were compared in terms of the shell or lorica oral diameter (LOD), analogous to gape size. We found 11 core and 49 occasional species. Diversity metrics were stable compared with shifts in abundances. Core species accounted for the majority of individuals in all samples. On each date, 9-22 occasional species, representing 10-15% of the population, were found. Species richness of the occasionals was positively related to population size. The identities of the occasional species found were unrelated to the time between sampling. The species abundance distribution of the occasional population was best fit by a log-series distribution, while that of the core species was best fit by a log-normal distribution. The species abundance distribution of the entire community was best fit by a log-series distribution. Most of the occasional species had LODs distinct from that of a core species and occupied size classes left empty by the core population. However, the most abundant and frequent of the occasional species had a LOD similar to that of a core species. Among tintinnids, which are planktonic protists, occasional species have a species abundance distribution pattern distinct from that of core species. Occasional species appeared to be composed of two groups, one of relatively abundant species and similar to core species, and a second group of ephemeral species with morphologies distinct from core species. The existence of two categories of occasional or rare species may be common: (1) those similar to, and thus perhaps able to replace, dominant species in the absence of a change in the environment; and (2) those distinct from dominant species and requiring different conditions to prosper.