Transgenic, virus-resistant papaya cultivars in Hawaii are easily identified by a colorimetric assay for the beta-glucuronidase (GUS) marker transgene.  We used GUS to track pollen movement from a central 0.5-ha plot of gynodioecious transgenic 'Rainbow' plants by seed assays on plants in surrounding border rows of non-transgenic 'Sunrise' papaya.  GUS evidence of cross-pollination occurred in 70% of female plants (43% of assayed seeds), compared with only 13% of the predominantly self-pollinating hermaphrodite plants (7% of seeds) segregating in the gynodioecious 'Sunrise' border rows.  The percentage of GUS+ seeds in border row plants showed a weak negative correlation (r = -0.32) with distance from the nearest transgenic tree (30 m maximum).  In a non-transgenic papaya field located 400 m downwind from the 'Rainbow' source, no evidence of GUS was found in 1000 assayed seeds.  In a separate study, the origin of GUS+ seed discovered in papaya fruits from an organic farm was investigated.  Leaf GUS assays revealed that 70% of trees were transgenic, indicating that the grower had planted genetically engineered seed.  The impact of pollen drift from transgenic trees within the same field was determined by screening for GUS expression in seed samples from 20 non-transgenic hermaphrodites.  Only 3 hermaphrodites (15%) showed GUS+ seeds, at low levels ranging from 3 to 6% of contaminated samples.  These data indicate that the major source of transgenic contamination in organic fields is seeds of unverified origin, rather than pollen drift from neighbouring transgenic fields.  Organic growers are advised to 1) plant only seed that is known to be non-transgenic, preferably obtained by manual self-pollination of selected non-transgenic hermaphrodites, 2) grow only hermaphrodite (self-pollinating) trees, removing any female or male plants from production fields, and 3) maintain 400 m distance from nearest transgenic papayas.