By studying the variation in morphometric characters of D. myceliophagus under changing environmental conditions, the most stable characters were found to be length of post-vulval sac, length of post-vulval sac as a percentage of total length, and length to vulva as a percentage of total length. A comparison of morphometric characters of D. myceliophagus and D. destructor under similar conditions confirmed their similarity but showed that since average length of post-vulval sac as a percentage of total length always differed between the two species they could be separated morphometrically. The implications of these results for selecting suitable characters for use in taxonomy are discussed.