A survey (2005-2006) of house fly, Musca dormestica L. (Diptera: Muscidae) populations on four Florida dairy farms demonstrated the presence of flies with acute symptoms of infection with salivary gland hypertrophy (SGH) virus on all farms. Disease incidence varied among farms (farm averages, 0.5-10.1%) throughout the year, and it showed a strong positive correlation with fly density. Infections were most common among flies that were collected in a feed barn on one of the farms, especially among flies feeding on wet brewers grains (maximum 34% SGH). No infections were observed among adult flies reared from larvae collected on the farms, nor among adults reared from larvae that had fed on macerated salivary glands from infected flies. Infected female flies produced either no or small numbers of progeny, none of which displayed SGH when they emerged as adults. Healthy flies became infected after they fed on solid food (a mixture of powdered milk, egg, and sugar) that had been contaminated by infected flies (42%) or after they were held in cages that had previously housed infected flies (38.6%). Healthy flies also became infected after they fed on samples of brewers grains (6.8%) or calf feed (2%) that were collected from areas of high fly visitation on the farms. Infection rates of field-collected flies increased from 6 to 40% when they fed exclusively on air-dried cloth strips soaked in a suspension of powdered egg and whole milk. Rates of virus deposition by infected flies on food were estimated by quantitative polymerase chain reaction at approximately 100 million virus copies per fly per hour. Electron microscopy revealed the presence on enveloped virus particles in the lumen of salivary glands and on the external mouthparts of infected flies.