SUMMARY Centrosome reduction is the decrease in centrosomal components during spermatid differentiation (spermiogenesis) [1, 2]. It is one of several dramatic subcellular reorganizations that leads to spermatozoa formation common to a wide range of animals [3]. However, the mechanism underlying centrosome reduction is unknown and its functions are unclear. Here, we show that in Drosophila melanogaster spermiogenesis, the quantity of centrosomal proteins is dramatically reduced [4, 5];for example, Asterless (Asl) is reduced ~500-fold and is barely detected in spermatozoa. Asl reduction is regulated through a subset of its domains by the master regulator of centriole duplication Plk4[6–8] and by the ubiquitin ligase that targets Plk4 for degradation: Slimb [9, 10]. When Asl reduction is attenuated by Asl overexpression, plk4 mutations, Plk4 RNAi, or Slimb overexpression, Asl levels are higher in spermatozoa, resulting in embryo’s with reduced viability. Significantly, overexpressing Plk4 and Asl simultaneously, or combining plk4 and slimb mutations, balances their opposing effects on Asl reduction, restoring seemingly normal fertility. This suggests that increased Asl levels cause the observed reduced fertility, and not other pleotropic effects. Attenuation of Asl reduction also causes delayed development and a failure to form astral microtubules in the zygote. Together, we provide the first insight into a molecular mechanism that regulates centrosome reduction and the first direct evidence that centrosome reduction is essential for postfertilization development.