Physiological memories of environmental stress can serve to predict future environmental changes, allowing the organism to initiate protective mechanisms and survive. Although physiological memories, or bookmarks, of environmental stress have been described in a wide range of organisms, from bacteria to plants to humans, the mechanism by which these memories persist in the absence of stress is still largely unknown. We have discovered that C. elegans transiently exposed to low doses of hydrogen sulfide (H2S) survive subsequent exposure to otherwise lethal H2S concentrations and induce H2S-responsive transcripts more robustly than naïve controls. H2S bookmarking can occur at any developmental stage and persists through cell divisions and development but is erased by fasting. We show that maintenance of the H2S bookmark requires the SET-2 histone methyltransferase and the CoREST-like demethylase complex. We propose a model in which exposure to low doses of H2S generates a long-lasting, epigenetic memory by modulating H3K4me2 modifications at specific promoters. Understanding the fundamental aspects of H2S bookmarking in this tractable system can provide mechanistic insight into how environmental exposures are translated into the epigenetic landscape in animals. Overall design: RNA was collected by Trizol prep from larval stage 4 (L4) animals. 4 samples of 3 treatment groups: Unexposed control, Naïve animals exposed to 1 hr high (>150 ppm) hydrogen sulfide, and Bookmarked (exposed to approx. 50 ppm hydrogen sulfide for approx 8 hr as embryos) exposed to 1 hr high hydrogen sulfide. Standard unstranded mRNA library prep and paired-end sequencing were used.