The mechanics of recruitment to species-specific populations of phytoplankton are reviewed. Importance is attached to distinguishing among physiological analogs of growth, increase in biomass, cell replication rate and net population changes, and to the correct separation of the expression of rate- and yield-limiting factors. Phytoplankton growth and cell recruitment are mediated by an internally controlled cycle in which the cell first accumulates the resources required to bring about an approximate doubling of its mass, then embarks upon the mitotic division of its nucleus, as the accumulated protoplasmic mass is allocated to the separating new daughter cells. Study of the recruitment of new cells in controlled, optimized laboratory culture conditions show cell replication to be a first-order process, best quantified by reference to the exponent of potential population change ( r′). Species-specific values of r′ are temperature-sensitive, generally achieving a maximum in the range 25–35 °C, perhaps permitting a population doubling within timescales of several hours to 2 or 3 days. Interspecific differences in temperature-specific replication rate are generally well-predicted by the morphometries of the organisms, especially, the surface-to-volume ratio of the algal unit (cells, coenobia, with or without mucilage).