arithmeticity of quaternionic modular forms on g_2 abstract: quaternionic modular forms (qmfs) on the split exceptional group g_2 are a special class of automorphic functions on this group, whose origin goes back to work of gross-wallach and gan-gross-savin. while the group g_2 does not possess any holomorphic modular forms, the quaternionic modular forms seem to be able to be a good substitute.  in particular, qmfs on g_2 possess a semi-classical fourier expansion and fourier coefficients, just like holomorphic modular forms on shimura varieties.  i will explain the proof that the cuspidal qmfs of even weight at least 6 admit an arithmetic structure: there is a basis of the space of all such cusp forms, for which every fourier coefficient of every element of this basis lies in the cyclotomic extension of q.