The linear gyrokinetic equation is solved in the local approximationfor a series of advanced stellarator configurations. Included in themodel are finite ion and electron temperature gradients and finiteplasma beta effects on general three-dimensional equilibria.This allows investigation of drift mode instability for core plasmaparameters 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 isan essential ingredient for microstability calculations.