A necessary first step in the analysis oJ the population genetics of a given species is the adequate description, both in space and in time, of the genetic variation present in different populations of that species. Such variation may be studied in terms of two principal differences. First, allelic variation at individual loci, and second. variation in the structure and/or the number of chromosomes. Both, in fact, frequently lead to the development of polymorphic facies. Those associated with allelic variation are now relatively well understood for they have pronounced, often dramatic, effects on the exophenotype, effects which greatly facilitate an analysis of their adaptive roles. Thus allelic polymorphism is associated with mimicry in butterflies, with camouflage in moths and snails, with resistance to malarial parasites in man, and with floral biology in primroses (Sheppard, 1967). By contrast, most of the known chromosome polymorphisms are exophenotypically cryptic in character. It is often assumed that such polymorphisms are concerned, in some undefined way, with biochemical effects (see for example Wallace, 196,8), but the balance of evidence to date is that chromosome polymorphisms are expressed initially in the endophenotype rather than the exophenotype (John and Lewis, 1966). Coupled with this, the endophenotypic effects they promote can be either supplemented or counteracted by other components of the genetic system. Their study and their analysis is thus both more complex and more exacting than that of the conventional allelic nolvmornhisms. It comes as no surprise, therefore, to find that in very few instances have we any real appreciation of the factors which govern the presence or the frequency of particular chromosome polymorphisms in particular populations. British populations of the mottled or club-horned grasshopper, Myrmeleotettix maculatus, are polymorphic with respect to the presence of supernumerary or Bchromosomes (Barker, 1960; John and Hewitt, 1965a, b). These extra elements have no obvious immediate effect on exophenotype, as has been demonstrated by a study of five populations with from 10% to 40% B-chromosomes, in which five morphological characters were scored for each individual. The characters were (1) the number of stridulatory pegs per hind leg, (2) average length of tibiae (3) average length of femurs, (4) average length of wings, and (5) number of segments per antenna. These particular characters were chosen since they are important in a range of functions, from courtship and species recognition to movement and dispersal, and are probably influenced by a variety of developmental pathways. Table 1 contains a brief summary of the results from these five populations, all of which occur in a parkland about 3 miles across. There is no consistent picture with regard to differences between non-B and B-containing individuals within or between populations. The reason for considering the between-individual and within-individual variances, is because of the established effect of B-chromosomes on the chiasma frequency. Individuals with such supernumeraries have higher mean cell chiasma