We investigated the effects of bacteria on Plasmodium falciparum sporogonic development in Anopheles stephensi. Four gram-negative (Escherichia coli H243, E. coli HB101, Pseudomonas aeruginosa, and Ewingella americana) and two gram-positive (Staphylococcus aureus and Staphylococcus epidermidis) bacterial strains were used in the study. Tenfold dilutions of bacteria suspended in phosphate-buffered saline were mixed with an infectious meal of gametocyte-enriched cultures and fed to adult mosquitoes. All gram-negative bacteria strains partially or completely inhibited oocyst formation at different concentrations. Additionally, geometric mean number of oocysts showed a correspondingly significant decrease with increasing bacterial concentration (P < 0.001). In contrast, gram-positive bacteria strains did not have any inhibitory effects on oocyst formation even at very high concentrations. Oocyst development was not affected by: (i) culture supernatants of E. americana, (ii) formalin-treated E. coli H243, (iii) lipopolysaccharide of E. coli J5 (mutant of 0111:B4). These studies show that gram-negative but not gram-positive bacteria affect sporogonic-stage development of P. falciparum in A. stephensi. Inhibition of parasite acquisition may be an attribute of specific or nonspecific cytoadherence properties of gram-negative bacteria to the parasites.