Abstract Various chemical thinning programmes were applied to the apple varieties Statesman, Dunn's Favourite, and Sturmer to test their effect on biennial cropping. Two chemicals were used, sodium dinitro-ortho-cresylate (DNOC) and alphanaphthaleneacetic acid (NAA). At appropriate concentrations both chemicals improved regularity of cropping when treatment was extended over several seasons. Results have been expressed as changes in a biennial bearing index and these show clearly the effect of individual programmes on the cropping habit. Effective chemical thinning treatments, continued for four to seven years, tended to increase total yield, and to increase individual fruit weights, except in light crop years. Fruit set records indicated that variations in response of Sturmer to regular NAA thinning sprays were not caused by any change in tree susceptibility induced by repeated treatment. No change in tree growth was noted on Sturmer receiving regular chemical thinning programmes.