Pachytene maps of chromosomes 16, 17, and 18 have been constructed and are illustrated diagrammatically and in comparison with photomicrographs of representative bivalents. Their relative lengths and arm ratios are similar to the corresponding values at metaphase. Bivalent 17 is characterized by overall pale staining relative to that of the remainder of the autosomal complement; it also exhibits a region of heavily compacted chromatin situated medially in its long arm, unlike 16 and 18, each of which has a compacted region adjacent to the centromere. Structural polymorphism was not observed during this study. Comparison between these maps and the corresponding diagrams (Paris Conference, 1971, Supplement, 1975) of metaphase G-banding provides a quantitative estimate of the increment of structural detail present at pachytene. There is a general compatibility between the two types of pattern, strongly suggesting that bands represent aggregates of adjacent chromomeres. For these reasons, and because chromomere patterns are of natural occurrence, pachytene is suggested as a reference standard for chromosome structural differentiation visualized at metaphase. A number of different methods are known to effect additional metaphase banding detail. Most of these are not widely used, even though there is need for finer detail in the visual or cytophotometric analysis of increasingly more subtle rearrangements of the genome. Some examples are given.