Individuals of Physa parkeri are capable of detecting the mucous trails of conspecific individuals, as well as those of Campeloma decisum. Upon encountering conspecific and heterospecific trails, P. parkeri both paused more often and turned more sharply. Frequency of pausing and mean turning angle did not differ between encounters with conspecific versus heterospecific trails. However, P. parkeri followed conspecific mucous trails more often than expected by chance, but tended to avoid trails of C. decisum. The response of the pulmonate P. parkeri to trails of the prosobranch C. decisum suggests that detection of existing mucous trails in nature may be a common phenomenon that transcends even large taxonomic differences.