Abstract The tintinnid ciliates community was monitored monthly over two years, from July 2001 to June 2003 at four stations in order to study the temporal and spatial patterns and the response of tintinnid species composition, abundance variation and seasonal succession to environmental variation in a chemical discharge zone in North Lebanon (Eastern Mediterranean). Diversity was estimated as the number of species and the Shannon Index. Tintinnid abundance showed regular marked seasonal changes with bimodal cycle in the stations not affected by the pollution discharge while the other stations showed differences in the timing of maximum abundance depending on the year. Density of tintinnids ranged between 1 and 231 ind. L−1 and the annual mean between 24 ± 27 and 37 ± 59 ind. L−1. The highest cell concentrations were observed in November and May of both years of the study in stations not directly affected by discharge and the minimal values were noted in February 2002 for all stations. The number of tintinnid species in samples ranged from 2 to 19 ind. L−1. Shannon Diversity index was between 0.67 and 3.26 bit. Spatially, there was significant difference (P < 0.05) among stations (in some seasons) for all the considered environmental variables except temperature, but no difference (P > 0.05) was observed among stations in the tintinnid abundance, number of species and diversity indices. The intermittent discharge of effluent and the deteriorated average conditions observed near the shoreline created a clear difference in a qualitative aspect, seasonal succession of populations and the most abundant species. Density and diversity of species were not related to any of environmental parameters (P > 0.01).