Juvenile steelheadOncorhynchus mykiss(formerlySalmo gairdneri) that were homozygous for two alternative alleles at the dipeptidase locus were released into stream sections of the Palouse River drainage, Idaho, to test the neutrality of the alleles with respect to survival or growth. After 4 months in the stream sections, juveniles homozygous for either of the two major alleles did not differ in mortality, length, or weight.