Although the resistance to fatigue crack propagation (FCP) is one of the most important design criteria for structural applications of materials, there have been limited studies on FCP behavior of magnesium alloys, particularly in the near-threshold region. In the present study, FCP behavior of an AZ91D magnesium alloy was investigated in conjunction with crack closure phenomena. FCP tests were carried out in air and high vacuum at ambient temperature under constant load amplitude. In order to determine the effect of crack closure on the FCP rates, the crack closure levels were measured using the compliance method. The AZ91D alloy showed much higher FCP rates in air than in vacuum, particularly in the low and intermediate Delta K regime. The crack closure levels were high in the near-threshold region and decreased with increasing FCP rates regardless of test environment. The crack closure measurements showed that the crack closure levels were higher in air than in vacuum, particularly at low FCP rates. The da/dN vs. Delta K-eff curves showed that the FCP rates were much faster in air than in vacuum, possibly caused by the moisture in air.