The study was concerned with the horizontal and vertical distributional patterns and the possible summer periodicity of diatoms epiphytic on eelgrass, Zostera marina L., in the vicinity of Gloucester Point, Virginia. Samples were collected in August 1963 and June and July 1964. Horizontal and vertical distributional patterns were demonstrated with respect to the change of total numbers of organisms present. The distributional pattern was, however, more pronounced in the vertical direction. The vertical comparisons generally indicated more pronounced growth of epiphytic diatoms at the blade tip than blade base. Horizontal comparisons generally show more growth in deep than shallow water, but there are exceptions. Two trends were present which suggested periodicity and a shift in generic dominance. Cocconeia and Navicula appeared to reach peak densities on or shortly before June, Whereas, Dimerogramma, Epithemia, and Nitzochia appeared to have reached a peak considerably before the June sampling date. The former two organisms maintained relatively high densities throughout the sampling period, whereas, the latter three did not reach a comparable average density until August. Of the eighteen genera identified, four were chosen for detailed graphical illustration in 1963 and 1964 because of their predominance throughout the sampling period. Cocconeis, Dimerogramma, Epithemia 2 and Navicula were predominant genera in 1963; Epithemia was replaced in abundance by Nitzschia in 1964. / Master of Science