The dominance of the diploid state in higher organisms, with haploidy gener
ally confined to the gametic phase, has Led to the perception that diploidy
is favored by selection. This view is highlighted by the fact that no know
n female organism within the Metazoa exists exclusively (or even for a prol
onged period) in a haploid state. We used fluorescence microscopy and varia
tion at nine microsatellite Loci to show that the false spider mite, Brevip
alpus phoenicis, consists of haploid female parthenogens. We show that this
reproductive anomaly is caused by infection by an undescribed endosymbioti
c bacterium, which results in feminization of haploid genetic males.