The mobilization of grassroots Catholic groups (base communities) in t
he Amazon region of Brazil provides a clear illustration of the influe
nce of religion on social activism. This mobilization has emerged in r
esponse to land conflicts and environmental problems that have resulte
d from the policies of the military government that ruled Brazil from
1964 to 1985. The Roman Catholic Church has responded on both the gras
sroots and institutional levels to these problems, but not always in a
consistent manner. The two case studies presented in this paper;show
the relationship between base communities and rural activism as affect
ed in different ways by patterns of ecclesiastical authority. The firs
t example illustrates how inconsistencies in support from the Church h
ampered the effective mobilization of small farmers to resist the loss
of their land to an aluminium refinery. The residents of the one rura
l village that successfully resisted expulsion from the land are suffe
ring from the effects of air and water pollution caused by the alumini
um plant and the threat of future health problems because of the proxi
mity of the company's toxic waste dump. The second example presents a
more positive outcome. It shows how a Church-supported organization of
small farmers in the region of the Transamazonic Highway empowered it
s members to defend their land rights, to obtain schools and medical s
ervices, and to develop their own plans for sustainable agriculture.