Secondary epidermal solitary chemosensory cells (SCCs) are widespread among the primary aquatic vertebrates. They resemble taste bud sensory cells in fine structure and may be innervated from facial or spinal nerves. According to previous studies, SCCs may constitute a water sampling system in the contexts of predator avoidance, habitat recognition and, in some cases, finding food. By quantitative scanning (SEM) and transmission electron microscopy (TEM) in 60 specimens (57 SEM, 3 TEM) of 16 developmental stages, from pre-hatchlings to adults, we describe the ontogenetic development of SCC densities and shapes of sensory apices in the zebrafish, Danio rerio. This is put into perspective with the ontogeny of external taste buds. Just prior to hatching, 3 days after fertilization (3d AF), sensory apices of SCCs penetrate between the squamous epidermal cells, whereas taste bud pores only appear at the onset of exogenous feeding (5d AF). SCC densities increase sharply from hatching shortly after metamorphosis (25d AF) up to 6 x 10(3) per mm2 on the head and remain relatively constant in density thereafter. Conservatively estimated, there may be approximately 3.2 x 10(5) SCCs on the head and 1 x 10(6) SCCs on the entire body surfaces of a zebrafish 180d AF. SCCs are spread evenly, but are 2- to 5-fold higher in density along the head than along the body. Sensory apices are brush-like in hatchlings and early juveniles, but tend to consist of a single villus in the adults. This ontogenetic change of SCC apices parallels the evolutionary change from 'oligovillous' cells in lampreys and elasmobranchs to the 'monovillous' SCCs in the advanced actinopterygian teleosts.