The sample size `n´ has been investigated with respect to the following fruit quality parameters of ‘Catherine’ cultivar peach: equatorial diameter, weight, skin colour, (co-ordinates CIE L*, a*, b*), firmness, dry matter content, acidity and soluble solids. The work refers to the years 2000 and 2002, for plants grown in the region of Murcia, on the rootstocks ‘Montizo’, ‘Monpol’, ‘Adesoto 101’ (‘pollizo de Murcia’ clones), ‘M.r.s. 2/5’ (mirabolán plum clone) and a common ‘pollizo’ (not selected). Regarding the calculation of sample size, two methods have been used, both based on the observed variability in the samples and on the acceptable error when comparing the sample mean with the estimated population mean. The results obtained with the two methods were similar. A great variability was seen for the values of sample size `n´, for each variable as well as among rootstocks and between years. For most parameters, the rootstock affected substantially the sample size: ‘pollizo común’, the standard rootstock and the “not selected” gave the highest sample size values for eight of the nine parameters studied. Of the remaining rootstocks, ‘Mr. S. 2/5’ also gave high values of `n´ for four parameters. Among the selected clones of ‘pollizo de Murcia’, a degree of homogeneity was seen for the values of `n´, which were lower. The observed values of `n´ are close or very similar to the values in the literature, which seems to justify that in previous reports the sample size `n´ varies according to the criteria of the different authors and that experience and knowledge of the different aspects of the proposed objectives are enough to allow sample size selection