Narrative:

I was cleared to rapid city as filed, climb to 5000 ft expect FL180 in 10 mins. After taxiing to runway 21, I was cleared for takeoff, turn right to 270 degrees. I departed runway 21 and reaching 1800 MSL I turned right to heading 270 degrees, contacted departure and told them I was climbing through 2300 ft for 5000 ft, heading 270 degrees assigned. I was informed by departure of mooney traffic at my 10 O'clock position, 5 mi at 6000 ft. The mooney was informed of traffic at 1 O'clock position 5 mi, a king air, climbing. At this time I was concentrating on looking for the traffic (climb rate 1000 FPM). A short time later departure control told the mooney his traffic was at 12 O'clock position and 1 mile. The mooney called the traffic in sight and that he was past. I looked behind me at about my 7 O'clock position and saw the aircraft behind me and above me. I did not call the traffic in sight. I continued to climb. Sometime later, I glanced at my altimeter and realized I was at 6000 ft. I reduced power to begin a descent back to 5000 ft. At that time departure called and asked my altitude. I informed him I was at 6000 ft correcting. Departure reminded me I had been cleared to 5000 ft, continue climb, cleared to FL180. Factors contributing: I left the autoplt off to be able to make a quick turn if needed. I was concentrating more on finding the traffic than climbing to an assigned altitude. After the traffic had passed behind me, I just continued to climb and forgot about the assigned altitude.

Related reports:

Google
 

Original NASA ASRS Text

Title: BE-9L PLT HAD AN ALT OVERSHOT, AND LESS THAN LEGAL SEPARATION DEPARTING FSD.

Narrative: I WAS CLRED TO RAPID CITY AS FILED, CLB TO 5000 FT EXPECT FL180 IN 10 MINS. AFTER TAXIING TO RWY 21, I WAS CLRED FOR TKOF, TURN RIGHT TO 270 DEGS. I DEPARTED RWY 21 AND REACHING 1800 MSL I TURNED R TO HDG 270 DEGS, CONTACTED DEP AND TOLD THEM I WAS CLBING THROUGH 2300 FT FOR 5000 FT, HDG 270 DEGS ASSIGNED. I WAS INFORMED BY DEP OF MOONEY TFC AT MY 10 O'CLOCK POS, 5 MI AT 6000 FT. THE MOONEY WAS INFORMED OF TFC AT 1 O'CLOCK POS 5 MI, A KING AIR, CLBING. AT THIS TIME I WAS CONCENTRATING ON LOOKING FOR THE TFC (CLB RATE 1000 FPM). A SHORT TIME LATER DEP CTL TOLD THE MOONEY HIS TFC WAS AT 12 O'CLOCK POS AND 1 MILE. THE MOONEY CALLED THE TFC IN SIGHT AND THAT HE WAS PAST. I LOOKED BEHIND ME AT ABOUT MY 7 O'CLOCK POS AND SAW THE ACFT BEHIND ME AND ABOVE ME. I DID NOT CALL THE TFC IN SIGHT. I CONTINUED TO CLB. SOMETIME LATER, I GLANCED AT MY ALTIMETER AND REALIZED I WAS AT 6000 FT. I REDUCED PWR TO BEGIN A DSCNT BACK TO 5000 FT. AT THAT TIME DEP CALLED AND ASKED MY ALT. I INFORMED HIM I WAS AT 6000 FT CORRECTING. DEP REMINDED ME I HAD BEEN CLRED TO 5000 FT, CONTINUE CLB, CLRED TO FL180. FACTORS CONTRIBUTING: I LEFT THE AUTOPLT OFF TO BE ABLE TO MAKE A QUICK TURN IF NEEDED. I WAS CONCENTRATING MORE ON FINDING THE TFC THAN CLBING TO AN ASSIGNED ALT. AFTER THE TFC HAD PASSED BEHIND ME, I JUST CONTINUED TO CLB AND FORGOT ABOUT THE ASSIGNED ALT.

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.