Electroporation conditions were optimized for the transfection of protoplasts isolated from an embryogenic cell line of sweet orange [Citrus sinensis (L.) Osbeck cv. Hamlin]. Electric field strength (375–450 V cm⁻¹), vector DNA concentration (100 μg ml⁻¹), carrier DNA concentration (100 μg ml⁻¹), electroporation buffer (pH 8), and pre-electroporation heat shock of protoplasts (5 min at 45°C) were optimized. The plasmid vector pBI221 containing the β-glucuronidase (GUS) coding sequence under the control of the CaMV 35S promoter was used and GUS activity was measured 24 h after electroporation. All variables significantly affected transfection efficiency and when optimal conditions for each were combined, GUS activity was 7714 pmol 4-methylumbelliferone (MU) mg⁻¹ (protein) min⁻¹. Protoplasts were then electroporated in the presence of green fluorescent protein (GFP) expression vectors pARS101 or pARS108. Green fluorescent embryos were selected, plants regenerated, and integration of the transgene was confirmed by Southern blot analysis. Both plasmids were constructed using EGFP, a GFP variant 35 times brighter than wtGFP, having a single, red-shifted excitation peak, and optimized for human codon-usage. pARS101 was constructed by placing EGFP under the control of a 35S–35S promoter containing 33 bp of the untranslated leader sequence from alfalfa mosaic virus. pARS108 was constructed similarly except sequences were added for transport and retention of EGFP in the lumen of the endoplasmic reticulum.