The lichen Peltigera membranacea is an association of two symbionts: the mycobiont, a heterotrophic ascomycete, and the photobiont, a Nostoc sp. cyanobacterium. The lec-1 gene, encoding a galectin-like protein, was identified in the metagenome sequence of the lichen, and analysis of the gene structure indicated that it was derived from the mycobiont. This was confirmed by transcriptomic analysis of freshly collected samples in which expression of lec-1 was detected in apothecia and rhizines, tissues that are composed of only fungal cells. Expression of lec-1 in portions of the lichen thallus that included both symbionts was up to two orders of magnitude greater than in tissues lacking photobionts; in contrast, expression of a housekeeping gene stayed relatively constant. These results show that lec-1 expression is influenced by the presence of the photobiont, consistent with a model in which recognition of photobiont ligands by lectins such as lec-1 is a factor in the establishment or maintenance of compatible symbionts.