In this paper we summarize the principal features of inheritance in the bacteriophage T2H, and describe some new experiments. The genetic structure of this virus has been analyzed in terms of mutational patterns (Hershey, 1946) and by recombination tests (Hershey and Rotman, 1949). These two types of evidence agree in showing that mutational changes occur in localized regions of a complex genetic system. Mutations producing different effects usually occur at different loci, but one example of multiple allelism has been found (Hershey and Davidson, 1951). In this instance, the locus of the alternative mutations could be analyzed rather completely because most of the host-range mutations selected in a particular way proved to belong to a single allelic series. It was found that one pair of distinct mutants satisfied all three criteria of allelism listed below, and that another pair satisfied none of them.