Studies on the cellular basis of learning and memory have revealed several distinct phases of memory that can be distinguished by their time course. In addition to the traditional short-term and long-term memory distinction, several other phases of memory have been identified: forms of intermediate-term memory, at least two seperable forms of long-term memory, and possibly several forms of short-term memory. This article presents the contributions made by research on phases of memory for habituation in the nematode Caenorhabditis elegans. Through behavioral, neural circuit and genetic analyses of habituation, research using this simple organism has provided insights into different memory phases. Studying experience-dependent plasticity of this behavior has not only provided corroborating evidence for the existence of short-, intermediate-, and long-term forms of memory, as have been demonstrated in both Aplysia and Drosophila, but also has revealed the possible existence of multiple forms of short-term memory.