The variability of morphometric characteristics, the structure of the seismosensory system and the external skeleton in females and nonspawning and spawning males of the spotted lumpsucker Eumicrotremus pacificus were studied. Females differed from males in their greater anteanal distance and stability of the structure of the external skeleton throughout the adult period of life. Spawning males are characterized by a significant change in a number of plastic characters and the structure of their external skeleton, apparently related to the features of the spawning biology of the Eumicrotremus species. No noticeable variability was revealed in the structure of the axial skeleton, the skeleton of unpaired fins, and the seismosensory system of the head of E. pacificus. Using the characters of external morphology and the external skeleton of females and nonspawning males in the systematics of the lumpfishes was confirmed as valid.