Phytophthora nicotianae is a common and destructive pathogen of numerous ornamental, agronomic and horticultural crops such as tomato, tobacco and citrus. Three monobranched C 13 alcohol alkoxylate non-ionic surfactants were evaluated for in vitro inhibitory activity against the different asexual structures of P. nicotianae. The same surfactants, labelled MBA1301, MBA1303 and MBA1306, were tested for their in vivo control capacity against P. nicotianae root rot of tomato (Lycopersicon esculentum) under glasshouse conditions. MBA1301, MBA1303 and MBA1306 differ in the molecular structure of the hydrophilic portion. The molecular weight of MBA1301 is comparable to that of MBA1303 and is eight times lower than that of MBA1306. The main in vitro activity for MBA1301 and MBA1303 was a direct lytic effect on the zoospores. Zoospore lysis was already observed in the presence of 1 mg ml À1 of these two surfactants and almost no zoospores survived an addition of 5 mg ml À1 surfactant. In addition, MBA1301 and MBA1303 reduced sporangia formation at a concentration of 5 mg ml À1 . Both surfactants only affect mycelium growth at concentrations as high as 100 mg ml À1 . MBA1306 did not show any effect on sporangia formation, zoospore release and mycelium growth of P. nicotianae at a concentration 10 times that of the other two surfactants. A good in vivo control of P. nicotianae on tomato in substrate culture was obtained for MBA1301 and MBA1303 whereas the control capacity of MBA1306 was significantly lower. The results of this research indicate that non-ionic alcohol alkoxylate surfactants can be used to control tomato root rot caused by P. nicotianae in substrate culture. In addition, the size rather than the arrangement of the polymers in the hydrophilic portion of the surfactant molecule determines the efficacy to control tomato root rot caused by P. nicotianae in substrate culture. r