Water samples were collected at four sites on the White River, Muncie, Indiana, on a biweekly basis from April 1980 to April 1981, to determine seasonal variation in A. hydrophila density. In conjunction with water sampling, temperature, dissolved oxygen, conductivity and pH were determined at each site. During this period a yearly mean of 589 colony forming units (CFU) per ml was obtained with a range of 0-6350. Peaks in mean A. hydrophila numbers occurred during the summer (1808 CFU ml-1) and early spring (3946 CFU ml-1) with lows occurring in winter (42 CFU ml-1). No significant correlations could be found between CFU values and the physico-chemical parameters studied over the calendar year. However, the compiling of data from May 1979 to April 1981 did yield a significant correlation between A. hydrophila density values and the following water quality parameters: temperature, conductivity, and dissolved oxygen.Fish were collected during June through October 1980 at various sites along the White River, Delaware County, Indiana, for the determination of past or present exposure to A. hydrophila infection by serological and direct isolation methods. Of 102 different fish sera tested, 36% (37/102) had a detectable titer to A. hydrophila (isolate #113). However, the direct isolation method only yielded seventeen percent (8/47) of the fishes sampled to be positive forA. hydrophila systemically. Agglutinins to A. hydrophila were demonstrated in eleven different fish species, whereas five different species revealed no detectable antibody to the specific particulate antigen employed. Estimated CFU ml-1 of A. hydrophila was the only parameter which correlated with the presence of agglutinating antibody to this organism in river fish populations.Ball State UniversityMuncie, IN 47306