ZT is a free software tool which performs simple and partial Mantel tests. Contrary to the simple Mantel test, which tests the correlation between only two matrices, the partial Mantel test investigates the correlation between two matrices while controlling the effect of a third one, and thus tries to remove spurious correlations. The zt software is written in the C programming language, both for speed of computation and for being able to handle very large matrices. The paper gives a general introduction of the simple and partial Mantel tests, describes the algorithm in detail and finally describes some concrete examples based on real datasets. The zt software is released under GNU's global public license.