The linear gyrokinetic equation is solved in the local approximation for a series of advanced stellarator configurations. Included in the model are finite ion and electron temperature gradients and finite plasma beta effects on general three-dimensional equilibria. This allows investigation of drift mode instability for core plasma parameters of a helical advanced stellarator (Helias) fusion power plant. Numerical results for a Helias reactor configuration are presented. It is shown that consistent treatment of beta effects on the geometry is an essential ingredient for microstability calculations.