THERE is growing evidence that adaptation may relate to precise conditions of an individual’s phenotype, though previous studies have concentrated on discontinuous variation(1). We show here that the copula duration of male dungflies, a widely cited example of optimality in biology(2-5), is tuned precisely and continuously to the size of the copulating male. During copulation, new sperm displace previously stored sperm at a rate that shows exponentially diminishing returns(6). Thus at the optimal copula duration a male’s paternity gains from continued copulation drop below that expected from searching for a new female. Small males have lower rates of sperm displacement(7); they also have less prospect of copulation by take-over of females from other males(8,9). Both effects predict that small males will copulate for longer, although displacement differences exert much the stronger influence. Observed copula durations are negatively correlated with size and concur with the optimality model across the natural male size range. This represents one of the first quantitative demonstrations of optimal responses covarying with a continuous aspect of phenotype. Phenotype-limited optimality profiles pose interesting evolutionary questions.