UNITED STATES

SECURITIES AND EXCHANGE COMMISSION
Washington, DC 20549

SCHEDULE 13G/A

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

PhaseBio Pharmaceuticals, Inc.

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

o Rule 13d-1(b)

x Rule 13d-1(c)

o Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

(a) Amount beneficially owned:

Rock Springs Capital Management LP: 1,314,374

Rock Springs Capital LLC: 1,314,374

Rock Springs Capital Master Fund LP: 1,275,000

(b) Percent of class:

Rock Springs Capital Management LP: 4.48%

Rock Springs Capital LLC: 4.48%

Rock Springs Capital Master Fund LP: 4.34%

(c) Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote:

Rock Springs Capital Management LP: 0

Rock Springs Capital LLC: 0

Rock Springs Capital Master Fund LP: 0

(ii) Shared power to vote or to direct the vote:

Rock Springs Capital Management LP: 1,314,374

Rock Springs Capital LLC: 1,314,374

Rock Springs Capital Master Fund LP: 1,275,000

(iii) Sole power to dispose or to direct the disposition of:

Rock Springs Capital Management LP: 0

Rock Springs Capital LLC: 0

Rock Springs Capital Master Fund LP: 0

(iv) Shared power to dispose or to direct the disposition of:

Rock Springs Capital Management LP: 1,314,374

Rock Springs Capital LLC: 1,314,374

Rock Springs Capital Master Fund LP: 1,275,000

Item 5. Ownership of Five Percent or Less of a Class

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following [X].

Item 6. Ownership of More Than Five Percent on Behalf of Another Person

See disclosure in Items 2 and 4 hereof. Certain funds listed in Item 2(a) are known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the Shares covered by this Statement that may be deemed to be beneficially owned by the Reporting Persons.

Item 7. Identification and Classification of the Subsidiary which Acquired the Security Being Reported on by the Parent Holding Company or Control Person

Please see response to Item 2.

Item 8. Identification and Classification of Members of the Group

Not Applicable.

Item 9. Notice of Dissolution of Group

Not Applicable.

Item 10. Certification

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.