In its native Europe, the lichen Lecanora conizaeoides is an obligate acidophile: it always grows on substrates with a low pH. Its notorious, rapid spread through Europe in the 20^th century followed an increase in atmospheric sulfur dioxide pollution that artificially acidified many substrates on which this lichen would normally never occur. During the last 50 years, this species has appeared in several scattered, coastal North American sites. In 2000, a new population was discovered in an Atlantic white cedar swamp in eastern Massachusetts; subsequently, three additional rural populations, as well as three suburban populations, have been found in the area. Specimens from these seven populations are morphologically and chemically indistinguishable from other North American and European specimens. Substrate pH measurements reveal that, with one exception, all substrates colonized by L. conizaeoides in eastern Massachusetts are highly acidic (pH < 4), thus demonstrating that this lichen is behaving as an acidophile, just as it does in Europe. At the four rural sites, L. conizaeoides displays an almost singular affinity for Atlantic white cedar (Chamaecyparis thyoides), growing on its wood and, less frequently, its bark. But at the three suburban sites, this lichen colonizes the bark of various tree species, most of which give pH values noticeably lower than at the rural sites. The acidification of these tree species at the suburban sites cannot be explained by high concentrations of atmospheric sulfur dioxide, however, because these concentrations have never approached the levels recorded in Europe during the spread of this lichen there. All our observations suggest that, in eastern Massachusetts, L. conizaeoides established itself first in rural Atlantic white cedar swamps, and is subsequently spreading to other, suburban sites which contain sufficiently acidic substrates. This pattern of invasion is strikingly similar to that hypothesized for Europe by Wirth (1985), who suggested that L. conizaeoides originated in Pinus mugo bogs, from which it gradually expanded into more urbanized areas.