Samples of LaMnO_{3 +δ} with 0.08 ⩽δ⩽ 0.16 have been synthesized in order to study the effect of non-stoichiometry, δ, which is related to the concentration of Mn^IV ions, on the physical properties of the material. For δ⩽ 0.10 the compound crystallizes in the orthorhombic perovskite-type structure, while for δ > 0.10 a rhombohedral cell is formed. All LaMnO_{3 +δ}samples with δ⩽ 0.12 are paramagnetic, or, below T_C ferromagnetic semiconductors, while samples with larger values of δ exhibit a discontinuity in the temperature dependence of the resistivity, indicating a transition to a ferromagnetic metallic-like state at low temperatures. Upon further lowering of the temperature the charge carriers become localized again. These results are consistent with thermopower measurements. All samples are ferromagnetic at low temperatures. The magnetic properties can be interpreted on the basis of Goodenough's superexchange model.