Laboea sirobila Lohmann, a marine oligotrich ciliate, was present in Long Island Sound, USA, from March through June 1984. Peak abundance occurred in mid-June, when it was estimated that 10.2 × 106 individuals m−2 were present in the 37-m deep water column. When preserved specimens were examined with epifluorescence microscopy, the photosynthetic pigments chlorophyll and phycoerythrin were observed within chloroplasts inside the ciliate. These pigments were also present in Myrionecta (formerly Mesodinium) rubrum Jankowski, a photosynthetic ciliate which co-occurred with L. strobila during the present study. Vertical profiles showed both ciliates to be most abundant in the upper few meters of the water column, especially during periods of strong thermal stratification.