Ferromagnetic (In,Mn)As/GaSb and (In,Ga,Mn)As/GaSb heterostructures with Mn composition x = 0.03-0.09 have been prepared by molecular beam epitaxy. It is found that the relation between maximum Curie temperature T-C and Mn content x can be expressed experimentally by T-C = 800x up to about T-C = 50 K within the limit of this study. The hole concentration sufficient for T-C = 50 K is found to be several times lower than the value shown in the existing theory (Science 287 (2000) 1049). Formation of structural pairs of Mn atoms (or few-atomic aggregates) in some close lattice sites is discussed to explain the observed results. (C) 2002 Elsevier Science B.V. All rights reserved.