ABSTRACTMotileSymbiodinium microadriaticum(Freudenthal 1962) were attracted to a variety of nitrogen‐containing compounds, including ammonium, nitrate, urea and some amino acids. No chemosensory response to phosphate, sulphate, vitamins, trace metals or sugars was evident. Motile algae responded to concentrations of ammonium, nitrate, and urea at least as low as 10−6M. High concentrations (≥ 10−2M) of ammonium appeared to inhibit attraction of motile algae. Calculations using ammonium release rates from various aposymbiotic hosts suggest that motileS. microadriaticumcan respond to released ammonium ca. 1 cm from the source.Cultured algae were not attracted to combined nitrogen cues for at least 2 days after inoculation into seawater with dissolved combined low nitrogen. Algae freshly isolated from starved animals were normally motile the day following isolation and attracted to ammonium and nitrate when maintained in seawater containing<1 μM ammonium and nitrate. The algae lost their ability to orient to nitrogen attractants the day after incubation into culture medium containing high levels of ammonium and nitrate. These results suggest that chemosensory behavior is suppressed when nutrients are present in the ambient medium or are stored by the alga.There were few differences in chemosensory abilities in different strains ofS. microadriaticumto the attractants assayed, suggesting that selection for a particular strain by a host species may not be due to differential chemosensory ability or cues. However, the absence of chemical attraction of motileS. microadriaticumto infected hosts may act to preserve strain selection occurring at other steps in the infection process of aposymbiot