Reproductive patterns in the lumpsucker fish Lethotremus awae were investigated by field observations and histological analyses. Males guarded egg masses in empty shells of the ribbed barnacle Balanus rostratus or in an empty hole, bored in offshore rocks by the boring bivalve Penitella kamakurensis, at 7-17 mm water depth. The number of egg batches per nest was ranged 1-5, and the number of eggs per batch was 66-133. Eggs shaped sphere with 1.6 to 1.9 mm diameter and adhered to each other. Histological observation on ovaries indicated that females are multiple spawners within a single mating season. The male first dorsal fin was enlarged and crown-like, being a conspicuous feature of sexual dimorphism.