Infection with Cryptosporidium is a leading cause of diarrheal disease and early childhood mortality. This apicomplexan parasite undergoes asexual and sexual replication within the same host, and recent studies have shown an intrinsic developmental program of obligate transition to male and female gametes and sex. While factors were identified that control male fate and development, how female gene expression is orchestrated remains largely unknown. Here, we use the Cryptosporidium Single Cell Atlas to discover an RNA-binding protein (F-RBP) as one of the earliest markers of female identity in C. parvum. Reporter parasites engineered based on this gene allowed us to calibrate transcriptional pseudotime against the real time of female development, revealing a significant window of transcriptional fate ambiguity. While F-RBP is an early transcript, the protein persists throughout female development and into the zygote. Conditional ablation of the F-RBP gene showed it to be dispensable for sex determination and early female development in vitro. However, the gene is essential in vivo, and its loss results in rapid cure. Cell biological experiments link this loss to the production of oocysts that release sporozoites incapable of host-cell invasion. F-RBP binds transcripts highly expressed in the female gamete, enriched for a sequence motif recognized by a Y-box protein family member, and forms mRNA-protein complexes in late females akin to processing or P bodies. We propose F-RBP's essential role to be in the regulation of long-term homeostasis of maternally inherited RNA required for sporozoite infectivity.IMPORTANCEThe parasite Cryptosporidium is responsible for millions of cases of severe diarrheal disease in young children, and the infection is both a consequence of and a risk factor for chronic malnutrition. There are no vaccines available, and a single approved treatment is largely ineffective. Infection relies on ingestion of the oocyst, an environmentally stable spore, which contains an invasive form of the parasite, the sporozoite. Oocysts and sporozoites are the products of parasite sex and the fusion of female and male Cryptosporidium parasites. Here, we report the critical role of an RNA-binding protein in female mRNA homeostasis. Maternal transcripts orchestrate the complex process of sexual replication and oocyst formation. Loss of the protein results in spores that fail to initiate the next round of the life cycle, and conditional ablation using a small-molecule trigger quickly cures infected animals, demonstrating the essential role of parasite sex in continued infection.