As in other apicomplexan protozoa, the ribosomal genes of Cryptosporidium parvum are present in small number and dispersed on several chromosomes. Two types of ribosomal small subunit genes have been identified in bovine isolates by sequencing and sequence-specific polymerase chain reaction. Using denaturing gradient gel electrophoresis, ribosomal gene expression was investigated in C. parvum oocysts and intracellular stages grown in tissue culture. In a calf-propageted C. parvum isolate, extra and intracellular stages expressed 2 types of transcripts. This observation contrasts with the differential expression of rRNA, genes in malaria species. Analysis of ribosomal gene expression in oocysts of a different genotype indicates that some C. parvum isolates express only l type of small subunit ribosomal RNA. These observations highlight fundamental differences in ribosomal gene expression among apicomplexan protozoa and within the species C. parvum and support the view that differential expression in malaria is an adaptation to a dixeneous life cycle.