The present invention relates to crystalline forms of (1aS,5aS)-2-(4-oxy-pyrazin-2-yl)-1a,2,5,5a-tetrahydro-1H-2,3-diaza-cyclopropa[a]pentalene-4-carboxylic acid ((S)-1-hydroxymethyl-2,2-dimethyl-propyl)-amide (Compound 1) and pharmaceutical compositions thereof that modulate the activity of the cannabinoid CB2 receptor and are therefore useful in the treatment of CB2 receptor-mediated disorders, for example, osteoarthritis; pain; hyperalgesia; allodynia; inflammatory hyperalgesia; neuropathic hyperalgesia; acute nociception; osteoporosis; multiple sclerosis-associated spasticity; autoimmune disorders; allergic reactions; CNS inflammation for example; atherosclerosis; undesired immune cell activity, and inflammation associated with a disorder selected from: osteoarthritis, anaphylaxis, Behcet's disease, graft rejection, vasculitis, gout, spondylitis, viral disease, bacterial disease, lupus, inflammatory bowel disease, autoimmune hepatitis, and type 1 diabetes mellitus; age-related macular degeneration; cough; leukemia; lymphoma; CNS tumors; prostate cancer; Alzheimer's disease; stroke-induced damage; dementia; amyotrophic lateral sclerosis; and Parkinson's disease.