Glucose 6-phosphate dehydrogenase (G6PDH) is an enzyme that plays a critical role in the production of NADPH. Here we describe the characterization of four transcripts (G6PDH-A, -B, -C, and -D) that putatively encode the enzyme in the southern cattle tick, Rhipicephalus (Boophilus) microplus. The genomic DNA sequence encoding G6PDH-A, -B, and -D in R. microplus spans around 14,312 bases in length and comprises 14 exons (exons Id, Ia or Ib thru exon XII). While G6PDH-C is predominantly encoded by exons II thru XII of this genomic sequence, we have yet to identify the location of the exon encoding the start of this transcript relative to the available genomic sequence. On non-denaturing polyacrylamide gels, more than one protein band with G6PDH activity was observed suggesting the presence of multiple isoforms expressed by the tick. The G6PDH-A and –C transcripts are present in larvae, unfed adult females and males, salivary gland tissues from fed adult females and males, and midguts of fed adult females. The G6PDH-B transcript, least abundant of the four in all stages and tissues analyzed, may encode a non-functional enzyme. G6PDH-D was only detectable in larvae, unfed adult females, and salivary gland tissues from fed adult females. We postulate that the differential expression of G6PDH-D may play a role in tolerance of oxidative stress that is induced upon feeding, and that the transcript abundance in fed females is a function of bloodmeal volume and the time adult females spend on the host relative to adult males.