We have used in situ hybridization to study the expression of the vertebrate hyaluronan synthase (Has) gene family members, designated Has1, Has2, and Has3, during mouse development. At embryonic day (E) 7.5, Has1 and Has2 are expressed throughout the gastrulating embryo. After E8.5, Has1 expression disappears, but Has2 continues to be strongly, albeit transiently, expressed in numerous tissues, including the branchial arches and craniofacial structures such as the palatal shelves and lens pit. Has2 is also expressed during cardiac, skeletal, and tail development. Has3 transcripts are first detected at E10.5 in the maxillary and mandibular components of the first branchial arch. Notably, Has3 expression in the developing teeth, vibrissae hair follicles, nasal cavity, and inner ear complements the expression pattern of Has2. Our results indicate that, whereas Has2 is exclusively expressed in some tissues, its expression pattern overlaps and/or complements that of Has1 and Has3 in others. Developmental Dynamics 233:130–141, 2005. © 2005 Wiley-Liss, Inc.