Previous estimates of the size of the Giardia lamblia genome have ranged from 30 to 80 million base pairs (Mb), based on DNA renaturation kinetics. This is much larger than the sum of the sizes of the 4 to 5 chromosomal DNAs seen in typical pulsed field gel electrophoretic analyses. One possible explanation is that each visible chromosomal DNA consists of several unresolved DNA species. To examine this we have performed quantitative densitometry of ethidium stained chromosomal DNAs and Notl genomic digests. We have also examined the distribution of rDNA on Notl genomic fragments. All of our results suggests that the true genome size is 10.6 to 11.9 Mb. It is conceivable that the previous larger estimates may be distorted by impurities in the DNA preparations used.