This report concerns variations in neuron number within the pyramidal cell layer of hippocampal regio superior in 18 inbred strains of house mice. There is a genetically associated variability in the total number of neurons in this pyramidal layer. Systematic strain variations in the orientation of the pyramidal cell layer are also present. Relations between the numbers of neurons in various experimenter-defined subdivisions of regio superior were examined following statistical corrections for the variations in orientation. This led to a preliminary delineation of 4 genetically-defined subdivisions of the regio superior pyramidal cell layer.