Parthenogenetic aphids contain two developmentally distinct populations of the intracellular bacterial symbiont Buchnera, one in the embryos and the second in the maternal tissues. The total number of bacteria is larger in the embryos than in maternal tissues for all apterous aphids from newborn larvae to mid-reproductive adults. Overall, the bacteria represented 2-5% of the total biomass of the symbiosis, and their density (in terms of bacterial: total protein) was higher in embryos (3-13%) than in maternal tissues (0.5-2.0%). The growth and division of the bacteria in the maternal tissues varied with aphid developmental age, with evidence for a restriction of bacterial division rates in fourth-instar larvae. The biomass and proliferation rates of bacteria may have considerable impact on the relative importance of embryonic and maternal symbiosis in the nutritional interactions between the bacteria and aphid tissues.