NMAC NEAR THE DAYTON; OH; ARPT BTWN ACR AND LIGHT ACFT. ATC APPEARED TO BE AWARE OF LIGHT ACFT; BUT GAVE NO ADVISORY. RPTR RECOMMENDS TCASII ON CARGO ACFT.

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WE WERE CLRED TO DSND TO 3000 FT WHILE TRACKING THE RWY 24R LOC. WE WERE ASKED TO CALL THE ARPT FOR THE VISUAL APCH. AT 5200 FT; AND STILL LOOKING FOR THE RWY; A SMALL PLANE (SIMILAR TO A CHEROKEE) POPPED OUT FROM UNDER THE NOSE; TRAVELING L TO R. WE TOLD THE CTLR WE JUST HAD A CLOSE CALL WITH A SMALL ACFT AND ASKED WHY IT WAS NOT CALLED OUT TO US. THE CTLR SAID IT WAS 'POP-UP' TFC. AFTER A LONG PAUSE THE CTLR STATED HE/SHE ASKED US TO LEVELOFF AT 6500 FT. NONE OF US HEARD SUCH A XMISSION AND WE WERE NOT DISTRACTED FROM THE RADIO DURING THIS TIME FRAME. IF THE CTLR KNEW TO HAVE US LEVEL AT 6500 FT THEN SHE MUST HAVE KNOWN THE ALT OF THE SMALL ACFT. IN ADDITION SHE MADE NO ATTEMPT TO RECONTACT US; IF THE ORIGINAL XMISSION WAS MADE; FOR OVER 1 MIN AS WE DSNDED TO THE SMALL ACFT'S ALT. I ASKED THE CAPT OF THE FLT Z BEHIND US. HE HEARD OUR CONVERSATION AND RECALLED NO REQUEST BY ATC FOR US TO LEVEL AT 6500 FT FOR TFC. IN ADDITION; THE CAPT OF THE FLT W; WHO WAS AHEAD OF US; HAD A SIMILAR SIT; WITH A SIMILAR ACFT; WITHIN 1/2 MI; AT HIS ALT; WHILE ON THE APCH. THE CTLR DID NOT CALL OUT THAT TFC EITHER; BUT WHEN ASKED BY THE CREW SHE KNEW ABOUT THE ACFT AND ITS ALT READOUT. RECOMMENDATION; TCASII FOR CARGO ACFT.