Navicula grimmei and Nitzschia palea occurring almost equally in a mixed population on department moist garden soil surface when maintained in fresh supernatant (of soil-water medium) at pH 7.0, temperature of 26 +/- 1 degree C and under continuous light (intensity of approximately equal to 30 micromol m(-2) s(-1)) in a culture chamber exhibited a similar cell survival period (of 28 d) and percentage (at the beginning 100 % and mid of survival period 65 %) and stop gliding 11 d prior to cell death (with gliding speed reduced in both from 204-330 microm/min at the beginning to 82.5-99 microm/min at the mid of gliding period) irrespective of their size differences. However, a sharp fall in the cell gliding period, gliding cell percentage and speed occurred at various levels (different from cell survival period and percentage) in both diatoms in a similar extent under water stress (2, 4 and 6 % agarized supernatant, liquid supernatant with 0.2-1.0 mol/L NaCl, blot-dryness of cells for 5-15 min), pH extreme of liquid supernatant (< or =5.0, > or =9.0), temperature extremes in liquid supernatant (< or =15, > or =40 degrees C), UV exposure (0.96-5.76 kJ/m2), lack of all nutrients from the medium (double distilled water), darkness or low light intensities (2 and 10 micromol m(-2) s(-1)), presence of 'heavy' metals (Ni, Cu, Zn, Co, Fe, Hg; 1-200 ppm), organic substances in liquid supernatant (DDT, captan, urea, 2,4-D, 100-2000 ppm; thiourea, 50-1000 ppm). N. palea sway (turn around at either ends) or not only when gliding but independent of cell gliding speed, which decreased continuously under all conditions.