Inoculations with the saprophytic fungus Trichoderma spp. are now extensively used both to promote plant growth and suppress disease development. The underlying mechanisms for both roles have yet to be fully described so that the use of Trichoderma could be optimised. We here show that Trichoderma asperelloides effects include the manipulation of host NO production. NO was rapidly formed in Arabidopsis roots in response to soil borne necrotrophic pathogen Fusarium. oxysporum and persisted for ~1 h but is only transiently produced ( ~ 10 min) when roots interact with T. asperelloides (T203). However, inoculation of F. oxysporum infected roots with T. asperelloides suppressed F. oxysporum initiated NO production. A transcriptional study of 78 NO modulated genes indicated most genes were suppressed by single and combinational challenge with F. oxysporum or T. asperelloides. Only two F. oxysporum induced genes were suppressed by T. asperelloides inoculation undertaken either 10 min prior to or post pathogen infection; a concanavlin A-like lectin protein kinase (At4g28350) and the receptor-like protein, RLP30. Thus T. asperelloides can actively suppress NO production elicited by F. oxysporum and impacts on the expression of some genes reported to be NO-responsive. Of particular interest was the reduced expression of receptor-like genes which may be required for F. oxysporum dependent necrotrophic disease development.