Among 68 strains of Citrobacter diversus isolated in a community hospital in a year and a half, six were characterized by late H2S production in iron-agar slants, while otherwise typical. Although not recorded in standard tabulations, the potential for Enterobacteriaciae to develop this quality by plasmid transfer appears to be virtually unlimited, and dictates caution in the use of failure to produce H2S as a criterion for exclusion. In the present group, the use of the adonitol reaction as a means of distinguishing these organisms from more typical H2S producers is emphasized.