I think this is an error in r-devel: R : Copyright 2003, The R Development Core Team Version 1.7.0 Under development (unstable) (2003-02-25) ... > x <- matrix(1:9,3,3) > class(x)<-c("a", "matrix") > class(x) [1] "a" "matrix" > class(x)<-class(x)[-1] > class(x) [1] "a" "matrix" # should be "matrix" > class(x)<-"matrix" > class(x) [1] "a" "matrix" # should be "matrix" Paul Gilbert