Primary cultures of gill cells from freshwater and seawater-adapted trout were compared. These cultures, developed from an explant technique, exhibited a similar growth. Ultrastructural comparison between cultured and in situ cells showed that most of the cells in primary culture resembled the so called 'pavement' cells, whereas chloride cells were not observed in the cultured epithelium. Several other cells types, representing a minority of cells in primary culture, were observed (mucous cells, vesicular cells, cells with large dense granules and cells containing lysosomes). Morphological observations of cultured pavement cells from freshwater and seawater trout gills were similar, although the density of cellular organelles in cells was less under freshwater conditions. In addition to the morphological comparison, the regulation of intracellular pH in cultured cells from freshwater and seawater gills was examined. Resting pHi was not different for freshwater or seawater gill cells. A sodium-dependent and amiloride-sensitive mechanism was found in cultured cells. Under the experimental conditions used here, this mechanism was most likely a Na+/H+ antiporter in pavement cells from freshwater and seawater-adapted trout. The comparison of pHi recovery after acidification of cells from freshwater and seawater gills showed that the activity or the number of antiporters was higher for cells from seawater trout gill.