A non-naturally occurring microorganism having a 1,3-BDO pathway is provided. The microorganism expresses at least one of the following 1,3-BDO pathway enzymes: an aldolase that catalyzes condensationof two acetaldehydes to produce 3-hydroxybutanal; and an aldo- ketoreductase, oxidoreductase, aldehyde reductase or alcohol dehydrogenase that reduces 3- hydroxybutanal to 1,3-BDO. The organism may further express one or more enzymes for producing acetaldehyde. A biosynthetic process involves condensing two acetaldehyde molecules to 3-hydroxybutanal using an enzyme from class aldolases; and selectively reducing 3-hydroxybutanal to 1,3-BDO using an enzyme belonging to the class aldo-ketoreductase, oxidoreductase, aldehyde reductase or alcohol dehydrogenase. The process can further include producing acetaldehyde by a biosynthetic method.