The coordinated transcriptomic responses of both mutualistic ectomycorrhizal (ECM) fungi and their hosts during the establishment of symbiosis are not well understood. This study characterizes the transcriptomic alterations of the ECM fungus Laccaria bicolor during different colonization stages on two hosts (Populus trichocarpa and Pseudotsuga menziesii) and compares this to the transcriptomic variations of P. trichocarpa across the same time-points. A large number of L. bicolor genes (>8,000) were significantly regulated at the transcriptional level in at least one stage of colonization. From our data we identify 1,249 genes that we hypothesize is the “core” gene regulon necessary for the mutualistic interaction between L. biolor and its host plants. We further identify a group of 1,210 genes that are regulated in a host-specific manner. This variable regulon encodes a number of genes coding for proteases and xenobiotic efflux transporters that we hypothesize act to counter chemical based defenses simultaneously activated at the transcriptomic level in P. trichocarpa. The transcriptional response of the host plant P. trichocarpa consisted of differential waves of gene regulation related to signaling perception and transduction, defense response and the induction of nutrient transfer in P. trichocarpa tissues. This study, therefore, gives fresh insight into the shifting transcriptomic landscape in both the colonizing fungus and their hosts and the different strategies employed by both partners in orchestrating a mutualistic