Loci encoding two very similar monomeric hemoglobins among the 10–12 larval hemoglobins of Chironomus tentans show opposite, apparently compensatory, changes in activity in the presence of a spontaneous regulatory mutant. The regulatory mutant maps near the structural loci for both hemoglobins in the right arm of chromosome 3. The locus showing a regulatory increase in rate of synthesis does so in both cis and trans alleles. The locus showing decreased activity, however, undergoes nearly complete inactivation in cis. The regulatory mutant may represent a lesion of a promoter serving the latter.