Direct (chromosome counting) and indirect (flow cytometry, stomatal size, chloroplast number of the guard cells and morphological observations) methods were tested in order to determine the ploidy levels of haploid and diploid watermelon plants of the cultivars Sugar Baby and Halep Karasi. The results revealed that all the techniques tested can be used successfully. It was determined that while counting chromosomes is cumbersome, producing plants for morphological observations requires a long time and flow cytometry is expensive and labour intensive. On the other hand, measurement of stomata and chloroplast counting methods are simple to use are less labour intensive and hence can be considered a practical alternative to the others. The data for the stomata in the haploids were length, 17-18 mu m; diameter, 10-12 mu m and number of chloroplasts of the guard cells, 6-7 and in the diploids they were 23-24 mu m, 18 mu m and 11-12, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.