SummaryTo study the effect of mutagenic/carcinogenic agents on P-element transposition, the P strains used should be denned, especially with respect to the number of intact and functional P elements present. In this investigation, the relation between the number of complete P elements present in dysgenic males and P-insertion mutagenesis was studied in severalMR(P) strains. The main conclusions from this investigation are: (1) Complete P elements can be present in the genome without genetic activity (even in a ‘dysgenic’ cross). As a consequence, the number of complete P elements present in particular dysgenic flies, is not necessarily an indication of their dysgenic genetic activity. (2) TheMR-h12/Cystrain carries two complete P elements, one on the X chromosome without and one on theMRchromosome with genetic activity (making this strain most suitable for studies on P-transposition mechanisms).