Puncture force was measured in `Gala'apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] fruit from 16 to 175 days after full bloom over 2 years using a range of circular flat-tipped probes (1 to 11 mm diameter) to test the firmness of each fruit. The area-dependent (Ka) and perimeter-dependent (Kp) coefficients of puncture force were determined and were used to calculate the indicative puncture force approximating a standard 11.1-mm-diameter Effegi/Magness-Taylor probe for even the smallest fruit. Ka declined exponentially throughout fruit development with much greater changes occurring closer to bloom. In contrast, maximum Kp occurred at 107 to 119 days after full bloom before declining progressively. Estimated firmness (using a 11.1-mm-diameter probe) declined constantly from 16 days after full bloom. Ka was associated with developmental changes in cortical tissue intercellular air space, cell volume and cell packing density although relationships changed throughout fruit growth. However seasonal change in Kp was not associated with any obvious anatomical change in the cortex.