The mobility of adult—flour beetles in their natural environment (a flour medium) was measured for 2 genetic strains and both sexes of Tribolium confusum. (When the sexes were observed separately, the mobility of a mutant and a wild strain did not differ in the males, but in females the mutants were nearly 10 times as mobile as the wild individuals. If males and females of the wild strain are observed together, the mobility of the males appears to approach that of the females.