Periodicity of microfilariae of Onchocerca volvulus in the skin was not believed to exist until investigators in Africa recently demonstrated diurnal periodicity. Interestingly, this maximal density of microfilariae in the skin has been shown to coincide with the peak biting time of the Simulium vector. The current study was performed as part of other studies in Guatemala. Maximal density of microfilariae in the skin was at 1000 hours or shortly thereafter. This peak is earlier than reported in Africa. The most active feeding period of Simulium ochraceum, believed to be the principal vector in Guatemala, had been previously shown to be from 0800 to 1000 hours. The current report lends support to other findings indicating that Onchocerca volvulus microfilariae have a diurnal periodicity. In addition, the occurrence of peak vector biting time at the same time as maximal numbers of microfilariae in the skin strengthens a hypothesis that these synchronous cycles have a biological significance.