When males of the Migratory locust are raised under infra-optimal temperature conditions, namely, with a rythm of 20°C night/28°C day instead of 20°C/33°C, they become sterile. The size of the testis is normal, gonial and meiotic divisions take place, and spermiogenesis is initiated. No ripe sperms are formed, however, and the spermiducts and seminal vesicles remain empty. It is demonstrated that it is spermiogenesis that is altered. The cells of a given cyst do not evolve synchronously as they should do and, in each individual cell, the highly ordered pattern of organelle differentiation is also disturbed. Almost all specialized organelles of germ cells are formed but some develop incompletely (axonemes missing their two central tubules), others are morphologically different from normal (nebenkern with very dense matrix), others are produced in extra copies (mitochondrial derivatives, flagella and centriolar adjuncts). The normal correlations between the states of development of the various organelles are disturbed and the topographical relationships also are distorted even when the individual organelles are morphologically normal. As a result, the spermatids never elongate properly, they never align, and they never form a bundle of ripe spermatozoa.