the olfactory organ in small and large juveniles of the tropical yellowstriped reef cardinalfish apogon cyanosoma, collected from the wild (total length [tl] range=17–32 mm, mean=25 mm) was examined by scanning electron microscopy. the olfactory organ consisted of two bilaterally radial rosettes in each fish. they were oval-shaped, located medioventrally, one in each of the olfactory chambers. in small juvenile a. cyanosoma the rosette comprised six lamellae, three on each side of a midline raphe while large juveniles had eight lamellae, four on each side. the lamellae had a continuous cover of cilia, except for the margins. in small juvenile a. cyanosoma (tl mean=24.3 mm) only microvilli were observed, whereas both microvilli and a few cilia, presumably from olfactory receptor neurons, were observed in large juvenile a. cyanosoma (tl mean=32 mm).