Parathion and methyl parathion are organophosphate pesticides whose US registrations have been cancelled by the EPA. Importing them for pesticidal use is prohibited; any import for research or other purposes requires EPA authorization under FIFRA, and CBP will coordinate with EPA at the port of entry.

Phosphite esters are derived from phosphorous acid (H₃PO₃) and have the general structure P(OR)₃ with phosphorus in the +3 oxidation state, while phosphate esters derive from phosphoric acid (H₃PO₄) with phosphorus in the +5 state. The oxidation state and the acid precursor determine which heading applies; the product's CAS number and structural formula are the definitive classification tools.

Yes. Endosulfan is listed under the Stockholm Convention on Persistent Organic Pollutants, and its production and use are banned or severely restricted in most countries including the US, where EPA cancelled all registrations. Imports are subject to EPA oversight, and trade in endosulfan for pesticidal purposes is effectively prohibited.