Mammoth Uncensored Word List:
56 - 7 letter words starting with is

isabels isagoge isatine isatins ischial ischium islands isleman islemen isleted ismatic isoamyl isobare isobars isobase isobath isochor isodica isodoma isodont isodose isoetes isogamy isogeny isogone isogons isogony isogram isogriv isohels isohyet isokont isolate isolead isoline isologs isomere isomers isonome isonomy isopach isopods isospin isotach isotone isotope isotopy isotron isotype isozyme issuant issuers issuing istanas isthmic isthmus