A technique for the demonstration of myofibrillar adenosine triphosphatase activity (ATPase) used for mammalian muscle has been modified to suit fish muscle. The mammalian method involves selectively inhibiting fiber types by preincubation at either alkaline (pH 10.4) or acid (pH 4.3) pH before incubation for myofibrillar (ATPase) activity. Fish muscle fibers were found to be generally inactivated under these conditions. Preincubation at an acid pH was found to be unsuitable for fish muscle because of the indiscriminate inactivation of the fibers. The effects of preincubating at pH 10.4 and incubating tissue sections for different time periods and at different pH's and temperatures have been investigated. A differential staining of fiber types correlated with biochemical data on myofibrillar ATPase for red and white muscles was obtained by preincubating sections for short periods (1–2 min) at pH 10.4. Under these conditions the intermediately positioned pink fibers were found to stain similarly to the white fibers of high myofibrillar ATPase activity. An investigation has been made of the qualitative distribution of fiber types in the myotomal muscle of live teleost species: coalfish (Gadus virens), grey mullet (Mugil cephalus), crucian carp (Carassius carassius), black mollie (Mollienesia sp), and glassfish (Chanda ranga). The pink fibers were found to be abundant in all the species examined with the exception of the glassfish.