C3HTN304, Mr=149.11, orthorhombic, P21212 ~, a = 9-107 (4), b = 9-600 (4), c = 6.383 (8)/~, V=558.1,/k 3, Z=4, D x=l.774gcm -3, MoK~, 2 =0.71069A,, ~t= 1.73cm -~, F(000)=312, T= 138K, R = 0.032 for 603 observed data and 0.041 for all 685reflections. The compound crystallizes as a zwitterion like many other a-amino acids. In the crystalline state, L-alanosine is a tautomer of the published structure so that the terminal position in the chain of the molecule is occupied by the N-nitroso group. Conformationally,L-alanosine shows close resemblance to L-aspartic acid, which explains its involvement as an analogous substrate in a number of L-aspartic acid utilizing enzymes.