In a field experiment on sandy loam, application of 0,0-diethyl 0-2 pyrazinylphosphorothioate ("Zinophos") 3 weeks after onions had been sown, gave a better control of attack by the stem nematode than treatments 10 days before or 6 weeks after sowing. The treatment was as effective as application of a contact nematicide that would have killed more than 99% of the nematodes. At low initial nematode densities the effect of the treatment on population density after harvest was comparable to that

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... a contact nematicide that would have killed 98% of the nematodes. This figure was higher the greater the initial density of the nematode. Of the onions that still looked healthy at harvest time 25%, 21% and 36% (in the order of the dates of treatment) developed symptoms of attack during storage. Application of 1.7 g per m 2 of 0-phenyl N, N' dimethylphosphorodiamide ("Nellite") at the dates mentioned above had only a slight effect on the attack of onions by the stem nematode.