Sirex noctilio F. is an exotic woodwasp now found in eastern North America where it shares natural enemies with native woodwasps of Pinus spp. To study the extent to which native hymenopteran parasitoids and parasitic nematodes could affect woodwasp populations, 60 Pinus trees with symptoms of S. noctilio attack were felled in 2007 and 2008 in Ontario, Canada. Each tree bole was cut into 1-m sections that were placed in individual rearing tubes; emergence was monitored from May to November of the year of felling. Female S. noctilio were dissected to assess parasitism by the nematode Deladenus siricidicola Bedding. Two species of Siricidae emerged from these trees; S. noctilio, which accounted for most of the specimens collected, and S. nigricornis F. Of the three species of parasitoid that emerged, Ibalia leucospoides (Hochenwarth) was the most abundant, accounting for an overall hypothetical Siricidae parasitism rate of almost 20%. This parasitoid emerged over a similar time period as S. noctilio—between early July and early September. Except in trees >15 m in height, parasitism by I. leucospoides generally appeared uniform throughout the bole. Parasitism rates did not vary between the 2 yr, but did between sites in 1 yr. Parasitic nematodes were found in the haemocoel of about one third of S. noctilio females dissected but were never found sterilizing the eggs; none were found in S. noctilio emerging from P. resinosa. These findings suggest that I. leucospoides is currently the primary invertebrate natural enemy of S. noctilio in Ontario.