Mire surface levels reflecting the water table depth are known to be an important vegetation gradient in boreal peatlands. We classified the vegetation by TWINSPAN, interpreted the vegetation gradients by ordination (Non-metric Multidimensional Scaling) and analysed the peat thickness for the TWINSPAN communities and species (Indicator Species Analysis) from small-sized sample plots over extremely thin-peated to medium thick-peated, minerotrophic mire expanse vegetation from an aapa-mire complex Hirvisuo, Northern Ostrobothnia, Finland. The TWINSPAN communities and the ordination-result could be interpreted in relation to the mire surface levels and additionally in relation to the stability of the water regime on the basis of water level measurements made from the same habitat types in an earlier research. Peat thickness correlated with the stability of the water regime so that communities with an unstable water regime on sand were thin-peated (0–30 cm), while communities with a stable water regime were more thick-peated. We conclude here that the mire-ecological water-table gradient was dissected into two vegetation gradients in Hirvisuo: the gradient formed by mire surface levels reflecting the mean water table level and the stability of the water regime, which represents a gradient of local importance over boreal Fennoscandia. Peat procuding ability of the bottom layer in mires seems to be much dependent on it.