Growth of regular echinoids, expressed as test diameter through time, generally shows a sigmoidal pattern. However, when urchin size is considered in terms of test volume we show that growth of the deep-sea echinoid Echinus affinis is ultimately linear, rather than saturating. We construct a simple allometric model of energy allocation that produces linear growth in volume in mature urchins by allocating an increasing proportion of net assimilate to reproduction. This model provides an excellent fit to the observed growth curve data. Data on gonad weight as a function of test diameter allow us to test the relationship between allocation to reproduction and urchin size predicted from the growth curve fit. Simultaneous fitting of the growth curve and gonad weight data allow us to consider a model where the allometry of net assimilation is allowed to vary. We investigate possible explanations for the fact that net assimilation appears to rise faster than linearly with weight in E. affinis. We conclude that strategic models of individual energetics provide a useful tool for the analysis of the limited data available on deep-sea populations.