Abstract: Foreign genes can be successfully expressed in the filamentous blue-green algae A nabaena sp. PCC7120 (A. 7120) at low levels. Changes in the physiological conditions of A. 7120 may affect expression of heterogenous genes in A. 7120. We described here that several stress factors induced the formation of short filaments (ca. 25 cells) from vegetative filaments of A. 7120. The data showed that red light and high temperature are efficient to induce the formation of short filaments; and after 48 h of red light induction, the ratio of the cell numbers of short filaments (CN_SF) to the total cell numbers of vegetative filaments (CN_T) is up to 85%. Meanwhile, DCMU is not efficient to induce the formation of short filaments, but the ratio of CN_SF to CN_T induced by red light supplemented with adequate concentration of DCMU in medium BG-11 can be increased slightly. High temperature at 45℃ treatment after 12 h can generate the highest ratio of CN_SF to CN_T about 87%. Furthermore, the exponential mid- or telo-phase growth of A. 7120 vegetative filamentous cells at 45℃ can easily form the short filaments. The photosynthetic analysis showed that the photosynthetic rates of oxygen evolution in short filaments are lower than those in normal vegetative filaments. It showed potential use of these A. 7120 short filaments with photosynthetic activities as recipient cells for expressing foreign genes.