The Bigmouth Sculpin, Hemitripterus bolini, is a cottoid fish in the family Hemitripteridae found throughout the Bering Sea and Gulf of Alaska to British Columbia, Canada, and northern California. Hemitripterids have internal gametic association with external fertilization as their mode of reproduction. With this strategy, spermatozoa are stored in the micropyle of eggs within the ovary after gamete transfer to the female and eggs are not fertilized until immersed in seawater. Female H. bolini deposit their eggs into at least 4 species of sponges. We collected eggs from Barrel Sponge (Halichondria lambei), Clay-Pipe Sponge (Aphrocallistes vastus), Boot Sponge (Acanthascus dawsoni), and Tree Sponge (Mycale loveni) in the Gulf of Alaska and Bering Sea. In contrast to its congeners, there has been little research on H. bolini, particularly any focusing on aspects of early life history stages. In this study, we describe the development of H. bolini eggs and larvae from newly-hatched stages to juveniles, and include general observations of osteology. The descriptions of larvae and juveniles are based on examination of 32 specimens, 5 of which were differentially cleared and stained to examine osteological development. Larvae hatch at a large size (13 to 14 mm SL) in an advanced stage of development. Postflexion occurs at approximately 20 mm SL, and transformation to the juvenile stage begins at about 31 mm SL. Larvae are heavily pigmented through late flexion, when pigmentation becomes reduced. Postflexion larvae and juveniles are also heavily pigmented. Ossification of most skeletal elements does not occur until postflexion. The description will help distinguish H. bolini from other hemitripterids and sympatric larvae of the family Agonidae that are similar in appearance but generally smaller and more slender-bodied at all developmental stages. The behavior of egg deposition in sponges, in conjunction with the use of sponges by other fishes such as juvenile rockfish and invertebrates such as Golden King Crabs (Lithodes aequispina) as refuge habitat, suggests that sponge grounds may provide essential fish habitat for H. bolini and other species.