<b>Abstract</b>— <p>The plain sculpin <i>Myoxocephalus jaok</i> being one of the most abundant members of the family Cottidae in the North Pacific region plays an important role in benthic communities. Its growth and longevity are compared by three independent operators in the populations inhabiting the northern part of its habitat, off the western coast of Kamchatka, and the southern part, in Peter the Great Bay, which are drastically different in terms of hydrological conditions. Significantly higher growth rates of females, their larger size and greater longevity than those of males, determined earlier, are confirmed for both populations. The age at maturity is identical in both areas, being 5 or 6 years for females and 4 or 5 years for males, after which the growth rate decreases. Differences in width and structure of the first annual increment in otoliths between populations are considerable primarily due to the different timing of mass hatching and the temperature differences in habitat conditions, which are particularly noticeable during the first years of life. Based on our estimates, in the first year of life, the Peter the Great Bay plain sculpins reach an average of 11.1 cm TL for females and 9.4 cm for males. Off the western coast of Kamchatka, sculpins reach an average of 6.1 cm long for females and 5.6 cm for males in the first year of life. In the western Kamchatka population, both males and females have a higher maximum age estimated by different operators: from 14+ to 16+ years for males and from 18+ to 20+ years for females, compared to from 11+ to 14+ years and from 17+ to 18+ years, respectively, for the Peter the Great Bay specimens.</p>