The distribution of Giardia lamblia (WB strain) trophozoites, encysting trophozoites (i.e., trophozoites bearing encystation-specific vesicles [ESV]) and cysts in the intestines of gerbils was examined over a period of 30 days. Trophozoites and encysting trophozoites were found in 3 equal length sections of the small intestine and in much lower numbers in the colon. Cysts were consistently found only in the second and third sections of the small intestine and in the colon. In a comparative encystation study in vitro, 4 populations of WB strain trophozoites were used: WB (uncloned), D1 (a recent clone of WB), WB-C6 (a 1983 clone of WB), and V1 (a recent subclone of WB-C6). The WB-C6 and V1 populations produced significantly higher numbers of encysting trophozoites and cysts in vitro than did the WB and D1 populations. In addition, WB-C6 and V1 produced significantly more water-resistant cysts in vitro than WB and D1. However, there was no difference in fecal cyst excretion in gerbils infected with the 4 populations of WB-derived trophozoites. One cycle of encystation/excystation in gerbils increased the in vitro encystation of WB and D1 populations but decreased in vitro encystation of C6 and V1. We could detect no difference in protein profiles or isoenzyme patterns between these 4 populations of cells before or after passage in gerbils.(ABSTRACT TRUNCATED AT 250 WORDS)