The complete nucleotide sequence of the exon of 654 base pairs encoding Pfs25 (the 25 000 MW surface antigen of P. falciparum zygotes and ookinetes) was determined in isolates from Brazil, Liberia, Ghana, Honduras, Malaysia and Indochina. In the Indochina isolate (Dd2) a guanine to cytosine transition at nucleoside 389 resulted in a substitution of alanine for glycine (Dd2 is a drug-resistant isolate with diminished capacity to infect mosquitoes). Isolate LF4 from Liberia (an isolate which does not produce gametocytes, and therefore does not produce Pfs25) had no deletions, insertions or base pair substitutions. No changes were found in any of the other isolates.