The spatial expression patterns of three genes [kelch-like protein homolog 10 (klhl10); armadillo repeat-containing protein 4 isoform X2 (armc4); mitochondria cytochrome oxidase I (mt-co1)] known to be important mechanistic components of spermatogenesis/spermiogenesis were localized in mantle tissue sections of male Mytilus edulis using nonisotopic in situ hybridization and RNA probes based on sequences derived from available mantle RNA sequencing libraries. The expression patterns showed that all three genes are primarily localized in a layer of cells adjacent to the outer wall of the testicular acini. This layer is known to contain developing stages of spermatogonia, spermatocytes, and spermatids. Kelch-like protein homolog 10 was expressed in immature spermatogonial cells/spermatocytes characterized by large round central nuclei and situated near the periphery of the testicular acini with little to no expression in adjacent populations of spermatids or mature spermatozoa at the center of the acini. Like klhl10, armc4 transcripts were also detected in the cells near the inner periphery of the outer acinar wall. In contrast, however, the armc4 expressing cells appeared to be spatially shifted more toward the inner edge of the cell layer and toward the center of the acini. The localization of cells expressing armc4 transcripts near the inner edge of the developing sperm layer in the testicular acini of M. edulis supports a role for armc4 in spermatid morphogenesis and maturation. Mitochondria-specific cytochrome c oxidase transcripts were localized in cells throughout the inner periphery of the testicular acini wall but not in association with mature spermatozoa in the center of the acini reflecting the significant level of cellular activity that occurs during early sperm development. This supports an important mitochondrial role in early and mid-spermatogenesis in M. edulis potentially related to sperm quality control. This work provides a first view into the molecular mechanisms behind spermatogenesis/spermiogenesis in M. edulis.