Mammoth Uncensored Word List:
90 - 8 letter words starting with is

isabella isagoges isagogic isarithm isatines isatinic ischemia ischemic ischuria islanded islander isleless islesman islesmen isoamyls isobares isobaric isobases isobaths isobront isobutyl isochasm isocheim isochime isochore isochors isochron isocline isocracy isocryme isodicon isodomon isodomum isodonts isodoses isoforms isogamic isogenic isogloss isogonal isogones isogonic isograft isograms isograph isogrivs isohyets isokonts isolable isolated isolates isolator isoleads isolexes isolines isologue isomeres isomeric isometry isomorph isonomes isonomic isopachs isophone isophote isopleth isopodan isoprene isospins isospory isostacy isostasy isotachs isothere isotherm isotones isotonic isotopes isotopic isotrons isotropy isotypes isotypic isozymes isozymic issuable issuably issuance isthmian isthmoid