The fungicidal activity of a series of aryltin compounds, Ar 3 SnOAc (Ar = m-Tol (mÀCH 3 C 6 H 4 ), 3,5-Xyl [3,5-(CH 3 ) 2 C 6 H 3 ], o-Tol (oÀCH 3 C 6 H 4 ) or Mes [2,4,6-(CH 3 ) 3 C 6 H 2 ]) and (Ar 3 Sn) 2 O [Ar = m-Tol, m-Anis (m-CH 3 OC 6 H 4 ), or o-Tol] as well as (Mes) 3 SnOH, for which IR and NMR ( 119 Sn) data are reported, has been assessed by radial growth assays on Aspergillus niger, Botrytis cinera, Mucor hiemalis, Fusarium solani and Penicillium chrysogenum, and the results are compared with those for the corresponding triphenyl-and tris (p-tolyl)-tin compounds. In general, sterically hindered systems (Ar = Mes) which are unlikely to achieve a trigonalbipyramidal five-coordinate geometry at tin with oxygen atoms in the axial positions, are ineffective as fungicides. However the o-tolyltin compounds, particularly the acetate, show some fungicidal activity. A larger size (m-Anis) or number (3,5-Xyl) of meta groups decreases fungicidal activity (to zero against P. chrysogenum) in comparison with (m-Tol) 3 SnX. Indeed, where test substances are inactive as fungicides, they promote the growth rate of P.chrysogenum by up to 60%. The steric effects implied by these data suggest that dimensions of active sites in the F 0 unit of the ATPase enzyme may differ significantly for each fungus studied. A model for the active site is proposed, based on the need of the Ar 3 Sn unit first to be able to reach the active site and then to occupy it with the required five-coordinate geometry so as to inhibit the activity of the ATPase enzyme.