Ischnocarpus exilis, a critically endangered cress, persists in one small population in a 1m2 section of a limestone tower in the South Island, New Zealand. The importance of intrinsic (seed production, dispersal ability and habitat specificity) and extrinsic factors (weed competition) acting to restrict the distribution of this threatened species were investigated with a field experiment, demographic monitoring and soil seed bank analysis. I. exilis is not confined to its present site because of high habitat specificity, but rather is limited by low seed production and limited dispersal as well as by competition with invasive weeds. Conservation management of this critically endangered species should focus on controlling weeds and on establishing new populations in suitable weed-free habitat. © 2002 Elsevier Science Ltd. All rights reserved.