Abstract. Let R be a ring with a monomorphism α and an α-derivation δ. We introduce (α, δ)-weakly rigid rings which are a generalisation of α-rigid rings and investigate their properties. Every prime ring R is (α, δ)-weakly rigid for any automorphism α and α-derivation δ. It is proved that for any n, a ring R is (α, δ)-weakly rigid if and only if the n-by-n upper triangular matrix ring T n (R) is (ᾱ,δ)-weakly rigid if and only if M n (R) is (ᾱ,δ)-weakly rigid. Moreover, various classes of (α, δ)-weakly rigid rings is constructed, and several known results are extended. We show that for an (α, δ)-weakly rigid ring R, and the extensions, the ring R is quasi-Baer if and only if the extension over R is quasi-Baer. It is also proved that for an (α, δ)-weakly rigid ring R, if any one of the rings R,is left principally quasi-Baer, then so are the other three. Examples to illustrate and delimit the theory are provided.AMS Subject Classification. 16S36. 16W60.1. Introduction. Throughout this paper R denotes an associative ring with unity; α is a monomorphism of R which is not assumed to be surjective; and δ an α-derivation of R, that is δ is an additive map such thatAccording to Krempa [18], a monomorphism α of a ring R is called to be rigid if aα(a) = 0 implies a = 0 for a ∈ R. A ring R is said to be α-rigid if there exists a rigid monomorphism α of R.The By [12], a ring R is α-rigid if and only if it is (α, δ)-compatible and reduced. Notice that the class of α-rigid rings and (α, δ)-compatible rings is a narrow class of rings, and it is easy to see that every (α, δ)-compatible ring is (α, δ)-weakly rigid; but there are various classes of (α, δ)-weakly rigid rings which are not (α, δ)-compatible, as we will see in Section 2.It is clear that every prime ring R is (α, δ)-weakly rigid for any automorphism α and α-derivation δ. In this paper we prove that for any positive integer n, a ring R is (α, δ)-weakly rigid if and only if the n-by-n upper triangular matrix ring T n (R) is (ᾱ,δ)-weakly rigid if and only if the matrix ring M n (R) is (ᾱ,δ)-weakly rigid.