Abstract. In the class of rank-1 transformations, there is a strong dichotomy. For such a T, the commutant is either trivial, consisting only of the powers of T, or is uncountable. In addition, the commutant semigroup, C{T), is in fact a group. As a consequence, the notion of weak isomorphism between two transformations is equivalent to isomorphism, if at least one of the transformations is rank-1. In § 2, we show that any proper factor of a rank-1 must be rigid. Hence, neither Ornstein's rank-1 mixing nor Chacon's transformation, can be a factor of a rank-1. N.B. In this paper 'transformation' shall mean a measure preserving transformation on a Lebesgue probability space. In general, we assume our measure spaces nonatomic; however the spaces that factors live on may be atomic. A transformation T is invertible if T" 1 exists and is a transformation. All sets mentioned are assumed measurable. In particular, S will be invertible if the T t are various powers of a single transformation.