A study was conducted to determine the prevalence of mastitis in pregnant beef heifers and the effect of this disease in dams on weaning weight of their calves during the subsequent lactation. The study was conducted in 30 crossbred beef heifers in Louisiana, USA from autumn 1994 (midgestation of the first pregnancy) to autumn 1995 (midgestation of the second pregnancy). Bacteriological analyses of quarter milk samples collected prepartum and during lactation showed that coagulase-negative staphylococci (CNS) predominated, followed by Staphylococcus aureus. The somatic cell count (SCC) averaged 436 000/ml from the uninfected quarters, whereas the mean SCC from infected quarters was 3 539 000/ml. The highest SCC was found in quarters infected with S. aureus, which averaged almost 6 000 000/ml. The overall percentage of quarters infected decreased from a high of almost 29% during midgestation of the first pregnancy to 25% during the prepartum period, and then to a low of 10% in early lactation. Dams with CNS mastitis weaned calves that weighed 15 lb lighter than uninfected controls, and dams with S. aureus mastitis weaned calves that weighed 23 lb lighter. At a market price of $0.85 per lb for 400- to 500-lb beef calves, it is calculated that producers may be losing over $18.00 per head or 5% of sales value due to this disease.