In this study we evaluated the biting deterrent effects of a series of saturated and unsaturated fatty acids against Aedes aegypti (L), yellow fever mosquito (Diptera: Culicidae) using the K &D bioassay module system. Saturated (C_6:0 to C_16:0 and C_18:0) and unsaturated fatty acids (C_11:1 to C_14:1, C_16:1, C_18:1, and C_18:2) showed biting deterrence index (BDI) values significantly greater than ethanol, the negative control. Among the saturated fatty acids, mid chain length acids (C_10:0 to C_13:0) showed higher biting deterrence than short (C_6:0 to C_9:0) and long chain length acids (C_14:0 to C_18:0), except for C_8:0 and C_16:0 that were more active than the other short and long chain acids. The BDI values of mid chain length acids (C_10:0 to C_13:0) were not significantly less than N, N-diethyl-meta-toluamide (DEET), the positive control. Among the unsaturated fatty acids, C_11:1 showed the highest activity (BDI = 1.05) and C_18:2 had the lowest activity (BDI = 0.7). In C_11:1, C_12:1, and C_14:1 BDI values were not significantly less than DEET. After the preliminary observations, residual activity bioassays were performed on C_11:0, C_12:0, C_11:1, and C_12:1 over a 24-h period. All the fatty acids (C_11:0, C_12:0, C_11:1, and C_12:1) and DEET showed significantly higher activity at all test intervals than the solvent control. At treatment and 1-h posttreatment, all fatty acids showed proportion not biting (PNB) values not significantly less than DEET. At 3-, 6-, and 12-h posttreatment, all fatty acids showed PNB values significantly greater than DEET. At 24-h posttreatment, only the PNB value for C_12:0 was significantly higher than DEET. The dose-responses of C_12:0 and DEET were determined at concentrations of 5–25 nmol/cm². As in the residual activity bioassays, the PNB values for C_12:0 and DEET at 25 nmol/cm² were not significantly different. However, at lower concentrations, the PNB values for C_12:0 were significantly greater than DEET. These results clearly indicate that mid chain length fatty acids not only have levels of biting deterrence similar to DEET at 25 nmol/cm² in our test system, but also appeared to be more persistent than DEET. In contrast, in vivo cloth patch assay system showed that the mid-chain length fatty acids, C_11:0, C_11:1 C_12:0, and C_12:1had minimum effective dose (MED) values greater than DEET against Ae. aegypti and their relative repellency varied according to species tested. The MED values of 120 (C_11:0), 145 (C_12:0) and 116 (C_11:1) nmol/cm² against Anopheles quadrimaculatus Say, indicated that these acids were not as potent as DEET with a MED of 54 nmol/cm². The MED ratio of the C_11:0 and C_11:1 for all three mosquito species indicated the C₁₁ saturated and unsaturated acids as more repellent than their corresponding C_12:0 and C_12:1homologues.