Six hundred and twenty pregnant women, living under conditions of year-round transmission of malaria in a rural coastal area of Madang, Papua New Guinea (PNG), were followed while attending mobile antenatal clinics and receiving chloroquine prophylaxis (300 mg base weekly). Whole blood chloroquine concentrations measured by ELISA from samples collected at delivery indicated a high level of drug compliance in regular attenders. Susceptibility is increased in primigravidae to Plasmodium falciparum but not to other malaria species, with the peak prevalence occurring at nine to 16 weeks gestation. The incidence of P. falciparum infection per person-month was 20% for primigravidae, 25% for those gravida 2, 17% for those gravida 3 or greater, and 14% for non-pregnant nulliparae. Some 8.7% of primigravidae and 9.5% of those gravida 2 had persistent infections. Prophylactic chloroquine is required in this pregnant population because of altered host immunity during pregnancy, but is reduced in efficacy because of chloroquine resistance. Despite this, a missed clinic attendance resulted in a two-fold increase in incidence for all pregnant women, indicating that chloroquine was having some effect.