In a study of the morphogenesis of respiratory syncytial (RS) virus, we have observed unusual tubular structures, both in infected and control HEp-2 cells. Tubular structures of similar morphology were described in HEp-2 cells infected with a type 1 foamy agent by Clarke, Attridge & Gay (1969). The fact that these tubular structures have been observed in uninfected control cells is something new to our knowledge and deserves mention.