CITATION S550 ACFT ON APCH TO UNCTLED ARPT AT DUSK BROADCAST THEIR POS SEVERAL TIMES ENTERING THE PATTERN WITH NO COM FROM ANY OTHER ACFT. 4 MI FROM ARPT RPTR FLC SAW INTRUDER ACFT COMING IN OPPOSITE DIRECTION AND HAD TO PITCH OVER TO AVOID THE ACFT WHICH APPARENTLY MADE NO EVASIVE ATTEMPT. NMAC.

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THE FOLLOWING INCIDENT OCCURRED DURING A FLT FROM OAK TO GRI. SHORTLY AFTER BEING CLRED FOR A VISUAL APCH TO GRI BY ZMP; I CANCELED MY IFR FLT PLAN AND BEGAN MANEUVERING TO ENTER THE TFC PATTERN AT GRI. MY FO BROADCAST OUR POS AND INTENTIONS ON THE CTAF FREQ SEVERAL TIMES DURING OUR APCH TO THE ARPT BEGINNING AT ABOUT 9 MI NW OF THE ARPT. THE ARPT CTL TWR HAD CLOSED FOR THE EVENING. DURING THIS TIME WE HEARD NO OTHER ACFT ON CTAF RESPOND OR RPT POS. AT ABOUT 4 MI NW OF GRI A SINGLE ENG CESSNA (A 152 OR 172 I THINK) APPEARED AT OUR 11 O'CLOCK POS COMING TOWARD US FROM THE OPPOSITE DIRECTION. IT WAS SLIGHTLY ABOVE OUR ALT. I PITCHED DOWN TO INCREASE THE VERT SEPARATION BTWN US. THE CESSNA THEN PASSED OVER US AND DID NOT APPEAR TO TAKE ANY EVASIVE ACTION. AFTER WE LANDED AT GRI AN EMPLOYEE AT THE FBO WHERE WE REFUELED TOLD ME THAT THE PLT OF THE CESSNA WITH WHICH WE HAD THE ENCOUNTER HAD JUST CALLED THE FBO FROM HIS ACFT BY TELEPHONE AND SAID 'ASK THE CITATION CREW IF THEY EVER SAW ME' OR SOMETHING TO THAT EFFECT. IT'S INTERESTING THAT A PLT WOULD BE COMMUNICATING BY TELEPHONE FROM HIS ACFT WHEN HE SHOULD HAVE BEEN MONITORING THE CTAF WHILE OPERATING ON OR NEAR THE ARPT FOR OTHER TFC AND ISSUING HIS OWN POS RPTING AS PER AIM CHAPTER 4; 4-9. IT MAY HAVE PREVENTED THIS INCIDENT. ALSO MY FO AND I WILL NEED TO MAINTAIN A HIGHER LEVEL OF VIGILANCE; ESPECIALLY WHEN DSNDING INTO AN UNCTLED FIELD AT DUSK AFTER A LONG HIGH ALT FLT. I WISH I HAD MORE INFO ON THE OTHER ACFT BUT I STILL DON'T KNOW IF HE TOOK OFF FROM GRI OR WHERE HE WAS GOING.