Postmarsupial development and growth of Natatolana borealis were studied from Skogsvåg and Raknesvåg on the western coast of Norway. Growth was determined from size and instar stage distributions. Growth was also examined in laboratory kept isopods that were collected from Skogsvåg. The postmarsupial instar stages were identified using the number of articles and the position of setae on the flagellum of the first antenna and other morphological features. The mean size of isopods in instar stages 1—3 did not show any significant seasonal variation in Raknesvåg. The estimated growth and mortality rates of males and females were not significantly different. Nine and 11 postmarsupial instar stages were found in material from Skogsvåg and Raknesvåg, respectively. The life span from instar stage 1 to 9 of male and female N. borealis from western Norway was estimated to be 3—4 y. Mortality rate increased significantly from instar stage 9 to 10 of both sexes. Isopods that attained instar stage 10 and instar stage 11 in Raknesvåg, may have lived 4—5 y. The mean size of instar stages 8 and 9 and the size-to-weight relationships were significantly higher in material from Raknesvåg relative to that from Skogsvåg and may have been caused by differences in food access and predation pressure at the two study sites. Specimens sampled from the Bay of Monaco attained smaller adult size than the isopods from the Norwegian population.