Zusammenfassung: Age-classes in crayfish populations can be determined from size-frequency data provided 4 tents are adhered to only crayfish collected from areas of rock substrate are used; trapping are excluded; species do not have longevities exceeding about 4 yr; and most importantly, sample sizes are greater than about 200. Studies that have failed to follow these guidelines have met with limited success in dissecting mixed frequency distributions. Further, 3 validation procedures must be undertaken: temporal replication within molting events to measure the precision of both data collection and size-frequency analysis; corroboration with known-age crayfish from either field recaptures or laboratory molt increments; and concordance with independent analysis of the mean size at onset of sexual maturity. When these checks support the size-frequency results, population age composition and analyses of rates of growth or mortality can be considered reliable.