In this work, the Cauchy problem for the semilinear Moore -Gibson -Thompson (MGT) equation with power nonlinearity |u| p on the righthand side is studied. Applying L 2 −L 2 estimates and a fixed point theorem, we obtain local (in time) existence of solutions to the semilinear MGT equation. Then, the blow -up of local in time solutions is proved by using an iteration method, under certain sign assumption for initial data, and providing that the exponent of the power of the nonlinearity fulfills 1 < p p Str (n) for n 2 and p > 1 for n = 1. Here the Strauss exponent p Str (n) is the critical exponent for the semilinear wave equation with power nonlinearity. In particular, in the limit case p = p Str (n) a different approach with a weighted space average of a local in time solution is considered.