The diet breadth and diet overlap of two co-existing species of flatfish, plaice (Pleuronectes platessa) and dab (Limanda limanda) inhabiting a shallow bay in south-west Ireland are compared. They were shown to vary with the resource matrix, the gut region and the sample size. Diet breadth was calculated using Levin's index and diet overlap calculated using Horn's index. Values of diet breadth were considerably less for numerical abundance data when compared to percentage occurrence data. Similarly, diet overlap values were less for numerically derived data. In dab, diet breadths were significantly lower for stomach-only contents than for intestine-only or whole gut contents. In plaice, there were no significant differences in diet breadths between gut regions. Significantly lower diet overlaps for stomach-only data were also found. Reducing the size of sample to N=30 and N=10 resulted in increases in the calculated value of diet breadth, by as much as 0·13 and 0·34 respectively, these increases were frequently significant. The same reductions in sample size for diet overlap decreased overlap values by 0·02 and 0·13 respectively, significant only when sample reduced to N=10.