This study describes the temporal fluctuations of doliolid populations sampled in the Bay of Villefranche-sur-Mer (Northwestern Mediterranean Sea) from 1967 to 1990. The abundance level of doliolids (strongly dominated by Doliolum nationalis and Dolioletta gegenbauri phorozooids) was estimated weekly according to Frontier's (1969) ordinal scale. Population abundance shows important interannual variations. The series displays a clear upward trend from the late 1960s to the early 1980s; thereafter, the abundances remain at a high and steady level. Five annual patterns of abundance level changes are identified, using cluster analysis. The influence of hydrological and meteorological conditions on doliolid abundance is tested using a Markov regression model for ordinal ecological time series (Menard et al., 1993). A reduction of salinity in the warm surface water due to the establishment of a halocline during early summer or autumn creates the main hydroclimatic conditions favourable to the development of the populations in question. These conditions mainly occurr during the oligotrophic period of the year, when the regenerated production system is the governing process.