ABSTRACT We present the only documented in situ observations of spiny lobster mating behavior. Four consecutive interactions, including copulation between a large male and smaller female Panulirus argus, were filmed in shallow Bahamian waters during May 1980. Mating activities were initiated by the male as he traversed the periphery of characteristic spiny lobster residences (e.g., coral reef crevices) openly in daytime, apparently searching for potential mates. The male approached and drew the female from shelter without force, by grasping her with his elongate second walking legs; he then pivoted on his rear walking legs and pitched backwards onto his dorsum, eventually positioning himself upside down below the embraced female. Copulation, and presumably spermatophore ejaculation, occurred when the male unfurled, then vigorously and rhythmically contracted his abdomen; copulation lasted less than one minute. Mating ended when the female tail-flipped under shelter, and the male righted himself, again approaching her. Antennular contact occurred frequently before and during copulation. The female participated actively by approaching the male over open sand. Prolonged pair formation was not evident. The film and our collective observations in large enclosures and in nature indicate that mating occurs diurnally between cooperative partners, which may copulate repeatedly.