Pecan [Carya illinoinensis (Wangenh.) C. Koch] fruit were thinned from `Mohawk' trees in Oklahoma and `Giles' trees in Kansas with a mechanical trunk shaker. All trees bore an excessive crop load before shaking. Fruit thinning improved the kernel percentage, individual nut weight, and kernel grade of `Mohawk', but nut characteristics of `Giles' were not affected by fruit thinning. Cold injury, caused by a sudden temperature drop in November, was positively related to the percentage of fruiting shoots in both cultivars. Fruit set in 1992 was negatively related to the percentage of fruiting shoots in 1991 in both cultivars. Consistent annual fruit set could be induced in `Giles' by fruit thinning, but return fruit set in `Mohawk', even at high levels of thinning, was low. Fruit thinning reduced yield the year of thinning in both cultivars. Thus, `Mohawk' trees should be thinned so that 50% to 60% of shoots bearing fruit at mid-canopy height would remain, and `Giles' trees should be thinned similarly to 65% to 70%.