A method is described for inducing motile forms of the Florida red tide organism,Gymnodinium breve, to convert to the sessile form. Available data indicates that the process is reversible. Key phases of red tide cycles must be the encystment‐excystment processes, and the formation of sessile forms represent a significant step in the understanding of the encystment process. The effect of light intensities and stray radiation from the cathodes of cool white lamps is compared and considered. Stray radiation from the cathodes of the fluorescent lamps appears to cause cell damage, as indicated by the first order rate constant kefor loss of motility. Deviations from the dependence of keon light intensity can be explained in terms of the distance from the point‐source radiation represented by the cathodes, and an inverse‐square law dependence is demonstrated.