Derivatives of the furocoumarin, psoralen, can penetrate intact cells or nuclei and cross-link opposite strands of the chromosomal DNA under the influence of long wave-length ultraviolet light. The potential of trioxsalen (4,5',8-trimethylpsoralen) as a probe for chromatin structure has been investigated. The DNA in both embryo nuclei and tissue culture cells from Drosophila melanogaster was found to be about 90% protected from trioxsalen binding relative to purified DNA. Digestion of trioxsalen-treated nuclei by micrococcal nuclease and gel electrophoresis of the resulting DNA gave the same type of band pattern that is characteristic of native, untreated nuclei are digestion. Nuclease digestion was therefore used to examine the distribution of bound trioxsalen in the DNA. The resulting DNA fragments were analyzed both by radioactivity measurements and quantitative electron microscopy. The nuclease cleaved intact photoreacted nuclei in such a way that preferential excision of trioxsalen containing regions of the DNA occurred, but, when acting upon purified DNA that contained bount trioxsalen, it attacked the trioxsalen-free regions preferentially. It was thus concluded that trixosalen binds at the sites corresponding to the regular nuclease-sensitive regions of the chromatin in nuclei.