Caenorhabditis vulgaris sp.n. (Nematoda: Rhabditidae), a necromenic associate of pill bugs (Armadillidium vulgare and Armadillidium nasatum) and snails (Oxychilus sp.), is described. C. vulgaris is known from two locations, both in the northeastern United States. It is gonochoristic, with males & females equally abundant. Its associations with pill bugs and snails are as dauer juveniles and appear not to be deleterious to the host animal. These associations are not a requisite part of the C. vulgaris life-cycle; cultures of C. vulgaris can be maintained indefinitely if grown on a bacterial lawn. C. vulgaris can be distinguished from other species of Caenorhabditis based on reproductive, molecular and morphological criteria.