A B737 FLT CREW ON APPROACH TO SAT EXPERIENCED A TCAS RA WHEN A VFR AIRCRAFT PASSED 300 FT BELOW HIM.

Narrative

TURNING R BASE TO FINAL FOR THE VISUAL APCH TO RWY 3 AT SAT. WAS AT 4000 FT MSL AND CLRED FOR THE VISUAL. STARTED A R DSNDING TURN. JUST AS THE DSCNT STARTED WITH ABOUT 750 FPM; I NOTICED AN ACFT ON TCAS SHOWING -400 ALONG MY DSCNT PATH. I IMMEDIATELY ROLLED OUT AND STARTED TO PULL THE NOSE UP. AS THE DSCNT WAS ARRESTED WE RECEIVED A TA WARNING IMMEDIATELY FOLLOWED BY AN RA COMMAND TO CLB. I FOLLOWED THE RA COMMAND AND SAW THE CONFLICT ACFT ON THE SCOPE FLOWING TOWARD US WITH ABOUT -300 ALT DIFFERENTIAL. WE CLRED THE TFC; REPOSITIONED FOR THE APCH AND LANDED NORMALLY. ATC IN THE TWR WAS INFORMED OF OUR DEV. THE TWR CTLR INFORMED US THAT HE HAD VFR TFC IN THAT LOCATION BUT THAT THE VFR PLT HAD INFORMED HIM THAT HE WAS AT 2500 FT MSL; EVEN THOUGH HE ACTUALLY WAS AT 3500 FT MSL. I THINK WE DID EVERYTHING CORRECTLY WITHIN OUR COCKPIT AND RESPONDED EFFICIENTLY TO THE TA/RA WARNINGS. IF THE TWR HAD INFORMED US ABOUT THE VFR TFC EARLIER; THIS EVENT MAY HAVE BEEN AVOIDED. HOWEVER; I THINK THE TWR DID NOT THINK HE WOULD BE A CONFLICT BECAUSE HE RPTED THAT HE WAS 1000 FT LOWER THAN HE WAS.