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The harvesting behavior of the ant, Messor barbarus, was studied in a mosaic grassland of the Mediterranean area. Among the 78 seed species naturally occurring on the study site, nine species dominated the ants’ retrieval in spring and in beginning of summer. Preferences of the ants were estimated from natural diet and related to the availability of seeds on areas used for foraging. M. barbarus preferentially harvested seeds of Sanguisorba minor, Hornungia petraea, plantain (Plantago bellardii, P. coronopus), catchfly (Silene gallica) and wild oat (Avena fatua), of which seedling recruitment may be altered by the foraging activity of ants. In contrast, seeds from Cerastium sp., Chrysanthemum myconis, Crepis sp., two clover species (Trifolium angustifolium, T. arvense) and four grasses (Aira caryophyllea, Agrostis pallida, Holcus lanatus, Brachypodium distachyon) were collected less frequently than expected from their natural availability. Two grasses (Agrostis sp and Radiola linoides) were unused by this harvester ant. Ants’ preference for a seed species could not be significantly related to the plant family, to the annual or perennial nature of the plant species or to seed productivity per crop. Only seed weight seemed to influence ant choices: seeds under 0.4mg were rarely retrieved, but 4-50mg seeds were harvested preferentially. This suggests that herbaceous plants living in this Mediterranean biotope might escape ant granivory by downsizing their seeds.