Testicular tissue removed from 10 month old fire-brats, Thermobia domestica Pack., was prepared for cytogenetic examination by the squash technique. Chromatin staining with acetic orcein and the use of phase contrast microscopy simplified preparatory procedures and preserved the greatest amount of cell integrity.

Photomicrographs taken of the complete meiotic cycle in the male indicated 34 chromosomes in reduction division -- 32 autosomes + X₁X₂. While all autosomes followed normal division patterns, the X chromosomes remained united by a chromatin strand and moved as a unit to one pole. Resultant products of this uneven division were daughter cells containing either 16 or 18 chromosomes.

All diads divided normally during the second phase. Thus, cells with 16 chromosomes produced daughter cells with 16, and cells with 18 chromosomes produced daughter cells with 18, respectively.

Spermatozoa produced by this maturation division were of two types -- those with heterochromatin and those without. Both are apparently viable, and no evidence of degeneration of either type was found.

All studies, thus far, indicate that in Thermobia individual spermatozoa are non-motile. Two must intertwine for approximately 1/3 their total length in order to produce translocatory movements.