To develop a new pesticide with phloem mobility, a series of new amino acid–fipronil conjugates were designed and synthesized based on derivatization at the 3-position of the pyrazole ring of fipronil. Experiments using a Ricinus communis seedling system showed that all tested conjugates were phloem mobile except for the isoleucine–fipronil conjugate, and that the serine–fipronil conjugate (4g) exhibited the highest concentration in phloem sap (52.00 ± 5.80 μM). According to prediction with log Cf values and uptake experiments with Xenopus oocytes, the phloem loading process of conjugate 4g involved both passive diffusion and an active carrier system (RcANT15). In particular, compared with for a previously reported glycinergic–fipronil conjugate (GlyF), passive diffusion played a more important role for conjugate 4g in the enhancement of phloem mobility. This study suggests that associating a nutrient at a different position of an existing pesticide structure could still be effective in obtaining phloem-mobile derivatives, but the distinct physicochemical properties of resultant conjugates may lead to different phloem loading mechanisms.