When ovipositing in Pinus radiata D. Don. the female of Sirex noctilio F. frequently drills two or more tunnels, which diverge from a common entrance hole in the bark. In these multiple tunnels eggs are normally laid in each tunnel except the final one where, instead, arthrospores of the symbiotic fungus, Amylostereum areolatum (Fries) Boidin, are deposited. When arthrospores occur in an egg tunnel this may be due to a "contaminated" ovipositor in which some arthrospores have remained from a previous act of egg and arthrospore deposition. In single tunnels, fungus is generally present but eggs are infrequent. The number of tunnels at one entrance hole appears to be related to the physiological condition of the tree, and single tunnels are relatively more common on healthy than on less vigorous trees. Thus the initial infection of healthy trees tends to be mainly with fungus and an associated secretion, which may weaken and predispose the tree to subsequent multiple tunnel attack when eggs are laid more abundantly. This might partly account for S. noctilio being more able than most siricids to attack healthy trees.