A 3-year study into the seasonal variation in the numbers of Boophilus microplus ticks infesting three breeds of beef cattle in Paraguay was carried out to provide the basic data necessary to formulate tick control recommendations. The effect of tick burden on weight gain was also analysed. Economic parameters for tick control were calculated. Half body counts of standard female B. microplus were carried out every 2 weeks, at the same time engorged female ticks were placed in the field to observe time to oviposition, larval hatching and 50% survival time. Cattle were weighed every 2 months. Peak tick burdens were encountered in late autumn, with a winter low. Observations of the non-parasitic cycle of the tick suggested that four generations year-1 were possible. High tick burdens were well correlated to low weight gains or weight losses, depending on the breed of cattle. Compensatory weight gains were seen in all three breeds at the end of winter when tick burdens decreased. The economic threshold for tick treatment was calculated at 53 and 54 ticks per animal in 2 separate years of the study. Recommendations for tick control programmes are given.