Many field and experimental studies have been conducted to reveal shifts in periphytic diatom assemblages under metal pollution. On account of their low cost, routine biomonitoring is based on approaches like community structure examination or biodiversity, which are not accurate enough in assessing metal contaminations. Several morphological traits have been tested like diatom cell lengths and abnormal forms frequencies. Diatoms were collected in winter 2004 on artificial substrates immersed along a gradient of polymetallic (cadmium/zinc) pollution in the Riou-Mort drainage basin (South West France). Significant reduction of the body size within the single taxa Gomphonema parvulum (Kützing) Kützing and Nitzschia palea (Kützing) W.Smith was observed in the polluted sites suggesting that metal-stressed environments select for smaller organisms. Abnormal forms were enumerated in highest percentages in the contaminated sites in April. This supported the hypothesis that heavy metals may interfere with silica uptake and metabolism.