Staphylococci were isolated from 103 out of 393 mastitic quarters of buffaloes (26.2%), of which 62 isolates (36.7%) were recovered from clini­cal cases and 41 (18.3%) from subclinical cases. S. aureus was isolated from 23.7% and 13.7% of quarters of clinical and subclinical mastitic cases, respectively. Studies of virulence factors of S. au­reus revealed that the majority of S. aureus iso­lates recovered from clinical mastitis (65%) grew as diffuse colonies on serum soft agar, indicating the presence of capsules, whereas almost the same percent of isolates from subclincal mastitis (64.3%) grew as compact colonies, which means they lacked capsules. Out of 40 S. aureus isolates obtained from clinical mastitic milk samples, 14 (35.0%) and 32 (80%) were positive for clumping factor and tube coagulase respectively, while out of 14 isolates recovered from subclinical mastitic milk samples, 9 (64.3%) and 10 (71.4%) isolates were positive for the same tests. 85.2% of the ex­amined S. aureus isolates were positive for hae­molysin production and the rate was higher among isolates detected from the clinical cases (90%) in comparison to that of subclinical cases (71.4%). 36 isolates (66.7%) were found to be toxigenic. Enterotoxin C was the most common as it was detected in 21 isolates (38.9 %), fol­lowed by enterotoxin A from 8 (14.8%) and en­terotoxin B from 7 (13.0%). Mortality rate was higher among mice infected I/P with S. aureus re­covered from clinical mastitic milk samples (73.3%), when compared with those infected with isolates obtained from subclinical mastitic milk samples (52.4%). The encapsulated isolates that produced toxins were highly virulent to mice. 100% mortality was caused by 12 isolates, of which 9 isolates produced enterotoxin C, 1 iso­late produced B and 2 produced A.