Described are novel reversible photochromic 3H-naphtho[2,1-b]pyran compounds, of the following graphic formula: ##STR1## wherein R.sub.1 is hydrogen or alkyl, R.sub.2 is hydrogen or preferably a carboalkoxy group and R.sub.3 is hydrogen or preferably an alkyl group, provided that either R.sub.1 or R.sub.2 is hydrogen, and B and B' are the aryl groups phenyl or naphthyl, a heterocyclic aromatic group or together form a spiro adamantylene group. Also described are organic host materials that contain or that are coated with such compounds. Articles such as ophthalmic lenses or other plastic transparencies that incorporate the novel naphthopyran compounds or combinations thereof with complementary photochromic compounds, e.g., spiro(oxazine) type compounds, are also described.