Poly(epsilon-l-lysine) (epsilon-PL) producer strain Streptomyces albulus PD-1 secreted a novel polymeric substance into its culture broth along with epsilon-PL. The polymeric substance was purified to homogeneity and identified. Matrix-assisted laser desorption ionization-time of flight mass spectrometry and nuclear magnetic resonance spectroscopy as well as other analytical techniques revealed that the substance was poly(l-diaminopropionic acid) (PDAP). PDAP is an l-alpha,beta-diaminopropionic acid oligomer linking between amino and carboxylic acid functional groups. The molecular weight of PDAP ranged from 500 to 1500 Da, and no co-polymers composed of l-diaminopropionic acid and l-lysine were present in the culture broth. Compared with epsilon-PL, PDAP exhibited stronger inhibitory activities against yeasts but weaker activities against bacteria. epsilon-PL and PDAP co-production was also investigated. Both epsilon-PL and PDAP were synthesized during the stationary phase of growth, and the final epsilon-PL and PDAP concentration reached 21.7 and 4.8 g L-1, respectively, in fed-batch fermentation. Citric acid feeding resulted in a maximum epsilon-PL concentration of 26.1 g L-1 and a decrease in the final concentration of PDAP to 3.8 g L-1. No studies on epsilon-PL and PDAP co-production in Streptomyces albulus have been reported previously, and inhibition of by-products such as PDAP is potentially useful in epsilon-PL production.