To investigate the significance of chemical and size stimuli for cocoon recognition and nursing behaviour inFormica cunicularia, nurses of this species were given a choice‐test between homospeci‐fic cocoons (Fc) and respectively:Latins alienuscocoons (La) which are much smaller than Fc, L. alienuscocoons (LaT) impregnated with a whole‐cocoon chemical extract of Fc;Formica lugubriscocoons (Fl) which are similar in size to Fc;F. lugubriscocoons (FlT) imbued with the Fc extract; and LaT cocoons. In this latter case the homospecific cocoons (FcT) were masked by whole‐cocoon chemical extract of La. As adoption criterion we used the life span of each kind of cocoon supplied at the choice‐test and tended by the ants for the following 13 days. La and LaT cocoons were never adopted but rapidly destroyed or eaten. On the contrary, FlT cocoons were tended by the ant‐workers in a manner similar to the Fc ones, whereas the Fl cocoons were generally destroyed. Finally, the FcT cocoons were considered alien and thus not adopted. These results clearly indicate that cocoon discrimination and care in this ant species is based on a synergistic perception of odour and size.