ABSTRACT Centrioles were studied in living and fixed testes of neuropteran insects by light, phasecontrast, and electron microscopy. Giant centrioles, measuring up to 8·0 μ in length (according to the species), develop in meiotic prophase, whereas only minute centrioles are found in spermatogonia. The giant centrioles have essentially the same ultrastructure as described for smaller centrioles, but their periodic structure is more clearly defined. The growth of giant centrioles consists of two phases. First, a procentriole lengthens to the size of its mother centriole, as in mitosis; then the four centriolar rods of one cell continue their growth simultaneously and at the same rate. At their distal ends centriolar vesicles are attached. These originate from invaginations of the cell membrane and apparently contribute to the growth of the centrioles. Neuropteran spermatids, like those of other insects, have only one centriolar rod, while spermatids of many other animals contain two centrioles. The origin and significance of this difference are unknown.