Many species of bacterial endosymbionts are acquired by animal hosts before birth, through direct transmission from mothers to eggs or embryos. This vertical transmission imposes a reduction in numbers or "bottleneck," and the size of this bottleneck affects the population structure and evolution of symbiotic lineages. We have estimated the size of the transmission bottleneck in Buchnera, the bacterial symbiont of aphids, using basic light and electron microscopy techniques. By serial-sectioning whole aphid abdomens, their eggs, and embryos, we determined the following parameters: (i) The average size of a Buchnera cell is 2.9 mm in diameter. (ii) The total number of Buchnera in an Acyrthosiphon pisum embryo was around 36700 whereas a first instar nymph contained more than 119000. (iii) The number of symbionts per bacteriocyte was around 800 in an embryo and 3200 in a first instar nymph. (iv) The total number of Buchnera transmitted to each sexual egg ranged from 850 in Nasonovia to 1800 in A. pisum to more than 8000 in Uroleucon ambrosiae. (v) The total number of secondary endosymbionts in A. pisum was 12170 for an embryo and 18360 for a first instar nymph. Secondary symbionts were arranged both extracellularly and in clusters of 2000-8000 bacteria inside bacteriocytes. These numbers are consistent with the few previous estimates of symbiont population sizes based on counts of gene copies.