We estimated populations of largemouth bass (Micropter salmoides) 12 inches tong or longer in total length by mark and recapture, using angling tournament catches on 28,200-acre Beaver Lake in 1974, and tournament catches and a supplemental creel census on 45,440-acre Bull Shoals Lake in 1975. Population estimates were 55,450 in Beaver Lake and 30,800 in Bull Shoals Lake. These estimates were in good agreement with those based on mark-recapture samples taken each spring by electrofishing near shore, and were correlated with harvest when applied to the lower reaches ofeach lake. Lakewide estimates ofanglers' catches indicated that the largemouth bass populations may have been 2-2.5 times larger than those based on tournament catches in hoth lakes. Probable sources of bias in these estimates include nonrandomization of marks and recaptures, and an exceptionally high tag return during the 1-month creel census on Bull Shoals Lake. Tournaments during periods ofpeak springtime fishing afford biologists a potential tool for efficiently estimating black bass populations in large impoundments. Under favorable conditions, tournament catches can provide 20-40% ofthe total fish required for a realistic population estimate.