Both juvenile and adult Helix aspera and H. lucorum showed significant periodicities in locomotor activity close to an imposed 24-h die1 of 13L: 11D, and to 20 diels of 13L:7D and 11L: 9D. Helix aspersa is active earlier in the night in 24-h diels than H. lucorum. In both 20-h diels H. aspersa showed a lag in attaining peak activity. We discuss the fact that H. aspersa copes better with 13L:7D and H. lucorum copes better with 11L:9D. Although both of them seem to grow faster in the 24-h diel, H. lucorum is indifferent to photoperiod in relation to its growth. Helix aspersa became mature in four months in all the three regimes, but its size and maturity were influenced by the three photoperiod regimes. Egg-laying was also influenced by the photoperiod regimes, but hatching was not.