Anacystis nidulans 6301 has been transformed in the light to ampicillin resistance with the plasmid pBR322. Permeaplasts prepared by 2-hr treatment of cells with lysozyme and EDTA are transformed with a 50-fold higher efficiency than that observed for cells. beta-Lactamase is present in A. nidulans transformed either with pBR322 or the plasmid pCH1 as evidenced by hydrolysis of the beta-lactam ring of Nitrocefin in extracts of transformants. beta-Lactamase also can be immunoprecipitated from extracts of [35S]methionine-labeled pBR322 transformants and coprecipitates with ribulose-bisphosphate carboxylase. Expression of the carboxylase is apparently amplified in pBR322 transformants as is that for several soluble proteins in pCH1 transformants. Chromosomal DNA per cell is increased about 6-fold after transformation of A. nidulans 6301 with either pBR322 or pCH1. A 4.3-kilobase-pair plasmid can be isolated from pBR322 transformants in addition to the endogenous plasmids pUH24 and pUH25.