Forces exerted by hyphae of the phytopathogen Pythium graminicola and mammalian pathogen Pythium insidiosum were compared with the mechanical resistance of their hosts’ tissues. Hyphal apices of both species exerted a mean force of 2 μN , corresponding to mean pressures of 0.19 μN μm −2 (or MPa) for P. graminicola, and 0.14 μN μm −2 for P. insidiosum. Experiments with glass microprobes showed that the epidermis of grass roots resisted penetration until the pressure applied at the probe tip reached 1–12 μN μm −2 . Previously published data show that mammalian skin offers even greater resistance ( 10–47 μN μm −2 ). Clearly, tissue strength exceeds the pressures exerted by hyphae of these pathogens, verifying that secreted enzymes must play a critical role in reducing the resistance of plant and animal tissues. It is presumed that hyphae are sufficiently powerful to bore through any obstacles remaining after enzyme action.