Solitary chemosensory cells (SCCs) are secondary sensory cells present in the epidermis of most primary aquatic vertebrates. In rocklings, the epidermis of the anterior dorsal fin (ADF) contains approximately 5 million SCCs. High-voltage electron microscopy and three-dimensional reconstructions from serial sections were used to examine the ultrastructure, arrangement, and synaptic contacts of the SCCs in the rockling ADF, Approximately 15% of all cells in the fin ray epidermis are SCCs, which occupy roughly 30% of the epidermal volume. These spindle-shaped cells are 25-30 mu m long and up to 10 mu m wide and terminate apically in a microvillus protruding 2-5 mu m above the epidermal surface. SCCs contain abundant endoplasmic reticulum and a large Gels apparatus in their proximal regions. The distal parts of SCCs contain characteristic vesicles, elongate mitochondria, and longitudinal strands of intermediate filaments. Synapses between SCCs and nerves resemble those found in teleost taste buds. One to four synaptic contacts per SCC were found. We hypothesize that the apparent secretory activity of the SCCs serves to replenish the apical membrane and mucus. Furthermore, parallel sampling of several hundred SCCs by single nerve fibers may serve low-threshold detection rather than stimulus localization.