The aim of the present paper is to study magnetosensibility and to seek for magnetic nanoparticles in ants. The social insects, by living in colonies, developed very efficient methods of nestmate recognition, being less tolerant towards individuals from other colonies. Therefore, any kind of strange behavior between nestmates and/or conspecifics, besides those present in their own behavioral repertoire, is not expected. The behavior study in the present paper analyze whether changes in the intensity of applied magnetic fields on Ectatomma brunneun (Smith) ants can cause changes in the normal pattern of interaction between conspecifics. A pair of coils generating a magnetic field was used to change the whole local geomagnetic field. Magnetometry studies were done on abdomens and head + antennae using a SQUID magnetometer. The results show that changes in the geomagnetic field affect the usual pattern of interactions between workers from different colonies. The magnetometry results show that abdomens present superparamagnetic nanoparticles and heads present magnetic single domain nanoparticles. Behavior experiments show for the first time that Ectatomma brunneun ants are magnetosensible. The change in nestmate recognition of Ectatomma ants observed while a magnetic field is applied can be associated to some kind of disturbance in a magnetosensor presented in the body based on magnetic nanoparticles.