Technical Abstract: The objective of the present study was to compare two new in vitro tests designed to evaluate the resistance level of R. microplus with two tests currently recommended by the Food and Agriculture Organization of the United Nations (FAO): one performed on larvae, the Larval Packet Test (LPT) and one performed on adults, the Adult Immersion Test (AIT). Two new tests were developed in our laboratories under the objective to reduce the labour intensity and to work with large sample numbers: the Larval Tarsal Test (LTT) and the Adult Contact Test (ACT). Both FAO- and the two in house tests are based on in vitro exposure of larvae or engorged females to serial dilutions of acaricide. However, the type of contact with the active ingredient (AI) differs among the tests. In the LPT, larvae are enclosed in filters soaked with active ingredient (AI) and forming a packet while the LTT is performed in microtiter plates and larvae have tarsal contact with the AI coated at the bottom of the wells. In the AIT, adults are fully immersed in the AI solution while the ACT ensures a full body contact of the adults with the AI, without immersion. A sensitive (Muñoz) and a resistant R. microplus reference strain (Ultimo) were selected and their susceptibility to 10 compounds from 6 classes (organophosphates, synthetic pyrethroids, formamidines, macrocyclic lactones, carbamates and pyrazol) was evaluated with the different tests. First evaluations showed that both adult tests require too many engorged females when the objective is to obtain a full dose-response mortality curve for several compounds. In-depth comparison therefore focused on the larval tests. It became evident that the LPT is far more labour-intensive and time consuming than the LTT, with which several compounds can be tested at the same time within one 96 well microtiter plate. The LTT is therefore more suitable for large-scale studies. Additionally, the LTT is capable to detect resistance with lower doses that the ones required by the LPT. Based on this comparison, the LTT is considered to be more suitable test to evaluate the resistance level of ticks and is currently used to evaluate resistance of field strains. Ongoing work is geared toward making this test independent from laboratory infrastructure and to develop it into a field test kit.