With a direct count assay, 10 fouling bacterial isolates have been characterized for their ability to adhere to glass cover slips and polystyrene dishes. Although most adhered in greater numbers to polystyrene, the preference was statistically significant for only seven isolates at the 95% confidence level, due in part to the greater variability in cell attachment to glass (coefficient of variation, 32.3% for glass compared with 10.0% for polystyrene). Employing polystyrene dishes, a novel microfouling assay was developed, based on the extraction and fluorometric determination of DNA. The assay was rapid, enabled the detection of as little as 0.15 mug of DNA per dish ( approximately 5,000 cells per mm), and showed good agreement with the direct count assay. The DNA method resulted in less variability among three replicates (average coefficient of variation, 7.06%) and allowed for estimation of bacterial density over a larger surface area per sample (1.89 x 10 mm) than was feasible with epifluorescence microscopy (0.06 to 0.1 mm).