Narrative:

Approach control cleared us (aircraft X) to 5000 ft and radar vectors direct to lit VOR and then heading 040 degrees. Shortly thereafter, approach cleared us to 4000 ft, heading 060 degrees. Approach control issued us a TA: C172 (aircraft Y) holding over the VOR at 4500 ft. We did not see the C172. As I was descending through 4500 ft, the TCASII went off. My first officer started calling out traffic's altitude and direction. I quickly descended to 4000 ft and turned left slightly. My first officer spotted the traffic at our 1 O'clock position. We were on converging courses, opposite 180 degree courses. Totally unsafe situation. I called approach control and advised them that we were too close for comfort. C172 pilot did the same. I called the tower supervisor once parked at the gate. He was going to investigate.

Related reports:

Google
 

Original NASA ASRS Text

Title: ATR72 FLC INITIATES EVASIVE TCASII ACTION TO AVOID A VFR C172 HOLDING AT LIT VORTAC. THE CTLR PREVIOUSLY ISSUED TA HOLDING AT VOR.

Narrative: APCH CTL CLRED US (ACFT X) TO 5000 FT AND RADAR VECTORS DIRECT TO LIT VOR AND THEN HDG 040 DEGS. SHORTLY THEREAFTER, APCH CLRED US TO 4000 FT, HDG 060 DEGS. APCH CTL ISSUED US A TA: C172 (ACFT Y) HOLDING OVER THE VOR AT 4500 FT. WE DID NOT SEE THE C172. AS I WAS DSNDING THROUGH 4500 FT, THE TCASII WENT OFF. MY FO STARTED CALLING OUT TFC'S ALT AND DIRECTION. I QUICKLY DSNDED TO 4000 FT AND TURNED L SLIGHTLY. MY FO SPOTTED THE TFC AT OUR 1 O'CLOCK POS. WE WERE ON CONVERGING COURSES, OPPOSITE 180 DEG COURSES. TOTALLY UNSAFE SIT. I CALLED APCH CTL AND ADVISED THEM THAT WE WERE TOO CLOSE FOR COMFORT. C172 PLT DID THE SAME. I CALLED THE TWR SUPVR ONCE PARKED AT THE GATE. HE WAS GOING TO INVESTIGATE.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.