Fixation of trout gills with different concentrations of glutaraldehyde has shown that the more concentrated solutions (25%) tended to produce clearer material for inspection under the scanning electron microscope. All gradations in surface sculpturing, from microvilli to microridges of about 13– 7–0 μm length have been observed, and their distribution on different parts of individual secondary lamellae is described. The possible significance of the types of sculpturing and their distribution is discussed in relation to the likely pattern of water flow across the secondary lamella during gill ventilation.