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year for which we had a sizable collection of oper- cles (spring 1978), we recognized four year classes: 1942, 1946, 1950, and 1969 (Table 3). In this collection, 32 of 79 fish were of the 1969 class (9 years old). By 1982, fish of the 1969 year class had become dominant, and had added four addi- tional annuli; they had 14 annuli in 1983, and 15 in 1984. Several fish from this group were also col- lected in 1975, 1976, and 1977. Other year classes that we were able to follow similarly were those of 1942, 1950, 1973, 1975, and 1976, Fish of the 1950 year class, which made up our second-largest collection, apparently dominated the population before the 1969 year class became dom- inant. Annual additions of annuli were followed as they were in the 1969 year class. In addition, we obtained (from the museum at the University of Nevada, Reno) seven cui-ui of the 1950 year class that were collected in 1956; other collections of fish of this year class were made in 1978 and 1982-84. 13 We had little difficulty with false annuli except in fish of the 1950 year class. This group had several false annuli within the first 6 years of life. To deter- mine which were the most plausible annuli, we compared opercles of fish of the 1950 year class with those of fish from other year classes. To our knowledge, the cui-ui 28-41 years old (Table 3) represent the oldest validated catostomids reported in the literature. However, catostomids in general are long-lived: Beamish (1973) validated white suckers (Catostomus commersoni) of age 17, and Minckley (1983) believed that razorback suckers (Xyrauchen texanus) in the lower Colorado River reached 30 years of age. The longevity of the cui-ui has undoubtedly had highly significant survival value for the species because of the scarcity of high- water years during which cui-ui were able to enter the river to spawn. Of the 11 year classes identified, 8 were hatched before the installation of MB dam, and during high-water years when spring flows Fig. 9. Cui-ui opercle bone, show- ing annuli (denoted by hatched line) from centrum to edge, and plane along which growth meas- urements were made.