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In the Cape Region of the Baja California peninsula of Mexico tumorous gall-like malformations (TGLMs) were found growing on the columnar cactus Pachycereus pringlei. This malformation has not been previously described. These TGLMs varied in shape (ball-like to fragmented), surface texture (smooth to uneven rough surface with many cracks), and size (1-70 cm diam.). Morphological analysis demonstrated that differently sized TGLMs represent different developmental stages. Organogenetically, TGLMs represent an anomalous teratogenous stem growth. The TGLMs appeared only in adult plants more than 3 m high. The fraction of plants bearing TGLMs in populations varied from 5 to 15%, however the malformations apparently did not disturb the population’s fitness to survive. The number of plants bearing small TGLMs (1-20 cm diam.) was greater than those bearing bigger TGLMs and the total number of small TGLMs dominated indicating that the appearance of TGLMs is at an early phase. The fact that the natural populations in which the TGLM-bearing plants occurred are widely distributed in the southern Baja California peninsula suggests that this malformation is widely spread and common in the arid region. (C) 1996 Academic Press Limited