(1) The allocation of mass, phosphorus and potassium to vegetative and reproductive structures was examined in six species of Viola (V. hirta, V. lutea, V. odorata, V. palustris, V. riviniana and V. tricolor) over one growing season at twelve sites. (2) The six species showed large differences in whole plant concentrations of P and K, both in their relative values and in the seasonal pattern of variation. (3) Mass allocation varied considerably between species. Allocation to roots ranged from 66% for V. riviniana in late winter to as little as 3% for V. lutea in summer, and for most species was lowest in summer, as it was, less predictably, for leaves. Root and rhizome allocation were generally complementary. (4) Peak mass allocation to sexual reproductive structures was generally low compared to most published figures, varying from 6 to 15%, except for V. lutea (35%). (5) P and K allocation varied greatly between populations, species, organs and times of year, and in many cases did not correspond closely to the pattern of mass allocation. Sexual reproductive structures mostly acted as sinks for P and K, except in V. lutea (with the highest mass allocation) and V. tricolor (the only annual studied). (6) P and K allocation to roots was very low, whereas leaves tended to accumulate K throughout the year, while becoming depleted of P in summer. Rhizomes generally accumulated P to a greater extent than K in autumn, suggesting different patterns of winter storage for P and K. (7) Differences in P and K allocation patterns between populations were at least as great as those between species for some organs, suggesting either considerable phenotypic plasticity or possibly the existence of ecotypic differences. (8) A fertilizer experiment on three species, involving both P supplements and K and N addition (the latter to induce a relative P deficiency), showed that allocation patterns were largely unaltered by the supply of P except where P was clearly deficient, when reproductive P allocation was increased. (9) The results suggest that, in these perennial Viola species, vegetative reproductive allocation is less variable than sexual allocation, which appears to be more dependent on resource availability, perhaps because of its greater element cost. Allocation to other structures appears to be less plastic. (10) The results demonstrate both the importance of defining the appropriate currency in studies of resource allocation and the wide range of allocation patterns that can be detected.