The reproductive cycles of Siphonaria denticulata and S. virgulata were determined by measuring the length or weight of the spawn of each species. S. denticulara lays a typical. coiled, gelatinous egg ribbon, cemented to the rocky substratum. S. virgulata lays a globular egg mass which is released directly into the water. This latter form of spawning is apparently unique amongst pulmonate molluscs. S. denticulata has a long breeding season, extending from November-December to the following April-May. Spawning usually occurs every 2 weeks for large adults, but smaller (i.e. younger) limpets spawn less frequently. Periods of spawning correspond to periods of full or new moons. The exact length of the breeding season of S. virgulata is unknown. Estimates of fecundity were obtained from counts of eggs within egg masses. There were differences in the fecundity of S. denticulata between localities, but S. denticulata and S. virgulata from the same locality had similar fecundities for similar-sized animals. Egg ribbons of S. denticulata experimentally placed high on the shore were found to suffer higher mortality from desiccation than those lower on the shore. This may explain why S. virgulata, which generally lives higher on the shore than its congener, has pelagic egg masses.