ABSTRACTOchromonas danica,a freshwater, planktonic chrysophyte, is capable of sensing the light conditions of its environment. This biflagellate alga has a swelling near the base of the short flagellum and a chloroplastidic stigma in close association with it. A procedure is described for the isolation of this three dimensional flagellar swelling, the presumed photoreceptor. In contrast to an earlier method developed for the isolation of the paraflagellar swelling fromEuglena gracilis,the protocol reported here forOchromonasresults in higher yields that should facilitate future biochemical investigations and could open avenues of investigation for the isolation and purification of the presumptive receptor protein. To verify the hypothesis that a rhodopsin‐like protein might be present in this alga, we applied a standard extraction procedure successfully used in the identification of retinal. We here report the purification and identification of all‐trans retinal inOchromonascells by column chromatography, HPLC and GC‐MS. Since retinal is the chromophore of rhodopsin‐like proteins, this finding may suggest that in these unicellular algae, too, a rhodopsin‐like protein could be the photorecepto