Let V be a group of operators
acting in fixed point free manner on a group G and suppose V has order relatively
prime to |G|. Work of several authors has shown that if V is cyclic of prime order or
has order four, G′ is nilpotent. In this paper it is proved that G′ is nilpotent if V is
non-abelian of order six, but that G′ need not be nilpotent for any further groups
other than those just mentioned. A side result is that G has nilpotent length at most
2 when V is non-abelian of order pq, p and q primes (non-Fermat, if |G| is
even).