An immunofluorescent method was developed to estimate the quantity and viability of Nocardia filaments in activated sludge and anaerobically digested sludge on both a mass and volume basis. The Gram stain counting technique of Vega-Rodriguez (1983) and Pitt (1988) was modified to estimate the mass of Nocardia in activated sludge and compared to the immunofluorescent method. Both methods were calibrated on a pure culture of chemostat grown Nocardia amarae. Using the immunofluorescence technique, Nocardia were estimated to comprise an average of 18% by weight of the VSS in a foaming activated sludge plant. Nocardia were found to be, on average, 79% viable as judged by INT reduction staining. Nocardia were found to comprise 13% of the VSS in a foaming anaerobic digester sludge and had an average viability of 63%. These organisms were estimated to be 30 to 50% viable in a mixed anaerobic digester with a hydraulic detention time of 14 days.