Phytolith analysis in archaeology suffers from a lack of taxonomic data on silicified forms in plant domesticates and related wild species. This study presents the results of a comparative analysis of phytolith size and morphology in maize (Zea mays L.) and wild grasses. Grass phytolith morphology is highly diverse. By combining the criteria of size and three-dimensional form of cross-shaped phytoliths, one can discriminate between maize phytoliths and those of wild grasses, including teosinte. This method for identifying archaeological maize was tested by analysis of samples from four sites in the Pacific watershed of central Panama. Phytolith spectra from deposits dated by radiocarbon to the early fifth millennium B.C. indicate the presence of maize. The interpretation of the phytolith data is supported by pollen studies.