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Feeding behavior and plant response to feeding were studied for the aphid Aphis gossypii Glover on susceptible and resistant melons (cv.Iroquois and TGR-1551,respectively).Average phloem phase bout duration on TGR-1551 was < 7% of the duration on Iroquois.Sixty-seven percent of aphids on TGR-1551 never produced a phloem phase that attained ingestion (EPG waveform E2) in contrast to only 7% of aphids on Iroquois.Average bout duration of waveform E2 (scored as zero if phloem phase did not attain E2) on TGR-1551 was <3% of the duration on Iroquois.Conversely,average bout duration of EPG waveform El (sieve element salivation) was 2.8 times greater on TGR-1551 than on Iroquois.In a second experiment,liquid nitrogen was used to rapidly cryofix leaves and aphids within a few minutes after the aphids penetrated a sieve element.Phloem near the penetration site was then examined by confocal laser scanning microscopy.Ninety-six percent of penetrated sieve elements were occluded by protein in TGR-1551 in contrast to only 28% in Iroquois.Usually in TGR-1551,occlusion was also observed in nearby nonpenetrated sieve elements.Next,a calcium channel blocker,trivalent lanthanum,was used to prevent phloem occlusion in TGR-1551,and A.gossypii feeding behavior and the plant’s phloem response were compared between lanthanum-treated and control TGR-1551.Lanthanum treatment eliminated the sieve element protein occlusion response and the aphids readily ingested phloem sap from treated plants.This study provides strong evidence that phloem occlusion is a mechanism for resistance against A.gossypii in TGR-1551.