The abundance of Hyalomma lusitanicum ticks in open areas of central Spain is the result of many natural and human factors. Control of tick populations by chemical and other means may be necessary until we can determine the key global factors in this important tick population. Despite many attempts to establish ecological control of ticks, there is little data about the activity of organic acids. This article describes tests on the in vitro toxicity of oxalic acid (OA) against wild H. lusitanicum adult ticks. Serial dilutions of OA dihydrate were prepared in distilled water with 1% of Tween 20. The treatment was applied in three replicates of 10-14 tick/dose from 0 to 1.037 mg OA/tick. Doses > 0.311 mg of OA killed all ticks in 24 h. Toxicity of the OA increased over time; therefore, the LD50 decreased from 0.22 mg at 24 h to 0.127 mg at 72 h. The results obtained show the toxicity of OA against adult H. lusitanicum ticks under in vitro conditions. Comparing toxicity in ticks and honey bees, OA seems a relatively safe treatment for nontarget arthropods and a potential alternative for tick population control. However, further research is needed to establish its real effectiveness and applicability under field conditions.