The interactions between aphids and their host plants seem to be analogous to those of plant-microbial pathogens. Unlike microbial pathogen effectors, little is known about aphid effectors and their ability to interfere with host immunity. To date, only three functional aphid effectors have been reported. To identify potato aphid (Macrosiphum euphorbiae) effectors, we developed a salivary gland transcriptome using Illumina technology. We generated 85 million Illumina reads from salivary glands and assembled them into 646 contigs. Ab initio sequence analysis predicted secretion signal peptides in 24% of these sequences, suggesting that they might be secreted into the plant during aphid feeding. Eight of these candidate effectors with secretion signal peptides were functionally characterized using Agrobacterium tumefaciens–mediated transient overexpression in Nicotiana benthamiana. Two candidate effectors, Me10 and Me23, increased aphid fecundity, suggesting their ability to suppress N. benthamiana defenses. Five of these candidate effectors, including Me10 and Me23, were also analyzed in tomato by delivering them through the Pseudomonas syringae type three secretion system. In tomato, only Me10 increased aphid fecundity. This work identified two additional aphid effectors with ability to manipulate the host for their advantage.