The genetic algorithm has been used in cryptanalysis. In this paper, we present a general attack to block ciphers by using this algorithm. This method considers the cipher as a black box and it is a known plaintext attack, whose objective is to find the key session. The crucial point of our approach is to make a partition of the key space into subsets determined by the division by a certain divisor, so that one can focus the attack in only some of these subsets. We show how this method can be successfully applied to a member of a block cipher family that contains the AES cipher.