ULLER (1927) and MULLER and PAINTER (1932) made the observation that the frequencies with which X-ray induced mutations occurred in regions of the X chromosome of Drosophila melanogaster fitted better the relative physical lengths of the regions than the genetic map lengths based on recombination. The frequency of mutation plotted as a measure of chromosome length they called a "mutation map" of the chromosome. The discrepancies between the genetic (recombination) and the mutation maps pointed out by them have been confirmed by others in the four decades since their major paper on the subject appeared (HARRIS 1929;OLIVER 1932;SPENCER and STERN 1948;IVES 1959).The discovery of the salivary gland giant chromosomes placed cytogenetic mapping on a far more precise physical basis than the ganglion metaphases available to MULLER and PAINTER (1932), DOBZHANSKY (1932) or HEITZ (1933). But extrapolation from polytene to meiotic stages (except for a common linear order) has been tenuous without a detailed knowledge of the growth of the giant chromosomes from the haploid chromosomes of the blastoderm. Some, as CHARLES i 1938), concerned with recombination frequencies. measured lengths of chromosomes on BRIDGES' (1938) map and assumed that they were proportional to meiotic chromosome lengths while others, as IVES (1959), used the number of BRIDGES' bands to estimate the number of loci in a given region.It is the purpose of the present paper to introduce DNA content as a basis on which to express the relative frequencies with which mutations are observed in different chromosome regions. Studies of early development in progress in our laboratory justify the assumption that the distribution of DNA along the X chromosome in salivary gland nuclei represents its distribution along the euchromatic portions of the meiotic chromosomes (RUDKIN 1964 and unpublished). This communication presents a preliminary "DNA map" of the X chromosome euchromatin giving the relative amounts of DNA between eleven cytogenetically mapped loci and compares the DNA map with "mutation maps" published for sex-linked lethals produced by X rays, by in situ H3 thymidine or P3? and by a mutator gene. A similar study of relevant data on the distribution of mutants induced by chemical agents is in preparation.The "apparent relative mutabilities" with respect to X-ray induced lethals of the DNA of seven regions covering all of the euchromatin of the X chromosome ' 1le~c.irch iuppoited IJ) Public llealth 5 e i r ~c e Hecealch (rranta h o ( A 01613 ( t o Jach Schaltz) and C A 06')17 and hy \dtional SI irnce I ounddtion Giant No G I8953