This study was designed to test Armothin as a candidate for chemical peach thinning. Fruit set on ‘Andross’ trees sprayed with 3% Armothin was lower than on those sprayed with 1.5% Armothin. Both 3 and 1.5% Armothin resulted in less fruit set than in the untreated control. The rootstock used did not influence the effectiveness of Armothin. However, when treated with 3% Armothin, the number and proportion of fruit set in the cultivar ‘Flavour Crest’ was much higher than that set in the cultivar ‘May Crest’. The highest percentage of fruit set per annual shoot ranged from 10 to 40% on ‘Katerina’ trees treated with 3% Armothin, whereas the range was 60–80% on the untreated control.