Rotavirus is a worldwide etiologic agent of diarrhea, responsible for large economic losses. We studied the seroprevalence of antibodies to group A rotavirus in cattle in 67 smallholder farms from Uruará municipality, using counterimmunoelectroosmophoresis with the NCDV strain as a standard antigen. Prevalence of positive smallholder farms was 95.6-100 percent. Significant differences were seen between age groups when the seropositivity rose from the youngest to the oldest groups and between females and males older than 1 year, when the seropositivity was higher in the first group