In most sessile marine invertebrates, metamorphosis is dependent on environmental cues. Here we report that heat stress is capable of inducing metamorphosis in the hydroid Hydractinia echinata. The onset of heat-induced metamorphosis is correlated with the appearance of heat-shock proteins. Larvae treated with the metamorphosis-inducing agents Cs+ or NH4+ also synthesize heat-shock proteins. In heat-shocked larvae, the internal NH4+-concentration increases. This fits the hypothesis that methylation plays a central role in control of metamorphosis. In the tunicate Ciona intestinalis, a heat shock is able to induce metamorphosis too.