The semi-drought-deciduous shrub, Diplacus aurantiacus, allocates a large, relatively constant proportion of carbon and nitrogen to sexual reproduction. Experimental manipulation at a site in the chaparral of coastal central California showed that both reproduction and vegetative growth were strongly limited by water and little affected by shade or by addition of nutrients unless accompanied by water. Potential competition for carbon between growth and reproduction is reduced by photosynthesis within reproductive structures; competition is also constrained by localization of translocation. Results are discussed in relation to the hypothesis that allocation to reproduction in Diplacus has been selected to maximize reproductive success.