RNA concentrations were determined on whole amphipods (Orchestia platensis) of different sizes and a relationship between RNA and dry weight established by least squares. The smallest (and fastest growing) individuals had ten times the RNA concentration of the largest. Growth rate in the amphipods was estimated by length‐frequencies in a time series and then related to RNA concentration. Growth rate and RNA concentrations were determined on a laboratory population of the brine shrimp Artemia salina. Growth predictions for A. salina made by using the growth‐RNA relation for amphipods were in reasonable agreement with the actual growth. Growth rate of larvae of the mud snail, Nassarius obsoletus, estimated from RNA determinations was consistent with observed values under laboratory conditions.These results indicate that the RNA‐growth relationship might be useful in estimating growth or productivity in other species or in mixed populations of animals.