Spiral ligament fibrocytes (SLFs) in the mammalian cochlear lateral wall participate in K(+) recycling; they are classified into five subtypes based on their morphology, distribution, and function. Regeneration of SLFs is a potential therapeutic strategy for correcting several types of hearing loss, prompting us to investigate how SLF subtypes are established during development. We compared transitional SLF-type marker expression with mitotic activity to evaluate proliferation-differentiation relationships in SLFs from postnatal rat cochleae. I.p. injection of 5-bromo-2'-deoxyuridine (BrdU) demonstrated that the overall mitotic activity of SLFs decreased significantly between postnatal day 7 (P7) and P10. For all developmental periods, BrdU incorporation was weakest in the area where type I SLFs reside. The onset of expression of markers for type II/IV SLFs followed the reduced mitotic activity of the cells, whereas that of aquaporin-1, a marker for type III SLFs, was already detectable at P7, when the type III SLFs were still proliferating vigorously. Distribution of BrdU(+) cells increased in the area of type I SLFs between P7 and P10, suggesting migration of SLFs from adjacent areas. We conclude that the time course of development of SLFs is subtype-specific.