Attention has been drawn to the low sex ratios for those aged 20-34 in the 1996 census of South Africa. Concern has been expressed that these low sex ratios are implausible and differ from what was found in earlier censuses. Some have suspected that they are an indication of the poor quality of the 1996 census overall. In virtually every country in the world, implausibly low sex ratios are found in the younger working ages, because geographically mobile young men of those ages are undercounted relative to women of the same age. When sex ratios by age are calculated based on those people born in South Africa and for which, as much as possible, similar geographic regions are included, a pattern similar to that seen in 1996 is found in all censuses since 1970. Like all national census offices, South Africa needs to seek ways to reduce the undercount of mobile young adults, especially mobile young men.